I use biscuit extensively everytime I join two boads edge to edge. Recently I started using them also in edge to face situations.All my reading about biscuits show contrasting opinions. Some swear tha biscuit make a joint stronger, others are equally adamant that non strength is added, they only help to keep the boards in good alignment.
Did anyone ever conducted a well done test to settle the question?
Thank you for educating us.
John Cabot
Replies
Dear John,
OK, this will prove to be fun. Of course it make the joint stronger. There is no way that it couldn't. I have been in construction for twenty years and the only miter joints that are virtually indestructible, have been biscuited. Look at it this way:
1) Once the biscuit swells, you have what amounts to a mechanical fastener.
2) You are increasing the surface area that will receive glue.
Some will argue that a properly glued edge joint is stronger than the surrounding wood. Done properly, that is true. A miter is an inherently weak joint, due to the fact that one is gluing end grain. Even if clamped correctly (Hartford or Chestnut Clam Clamp) it will break fairly easily. Biscuit it and clamp it and it becomes incredibly strong. Try it.
Best,
John
I don't know about any tests but I burn wood to heat my place. All the scrap from the shop and job sites goes in the kindling box. I've had several pieces of edge glued panels with biscuits in the box. I split them up with an axe and try to split them at the joint. A good number of the biscuits I have split open, have not swelled enough to make a bond. I'm a nut when it comes to glue application. I make sure the slots and the biscuits are fully coated.
I also had a problem with a wide staircase landing. It was too large to put in as one piece. I used biscuits in the joint in addition to edge gluing. I couldn't get clamping pressure on the joint and hoped the biscuits would help hold. When the heat went on, the panel shrunk and opened cleanly right at the joint. You can see the biscuits didn't hold.
I think biscuits are a great alternative to stub tenons, IF, there is no downward or pulling force on the joint. They add stability and help with alignment on something like a kitchen cabinet face frame, webbing, slipped into a miter. Places where you could almost get by with a butt joint but want to lock the pieces together, so a lateral force doesn't push the pieces individually. A wooden nail, basically. Other than these applications, I think they are a waste of time. You wouldn't build a cabinet with nails, would you?
Biscuits are not a substitute for good joinery practices and they add little in applications like edge gluing. In a face to edge situation, they can help in positioning the piece but they won't have the strength to hold them together, long term. For finish carpenters, a biscuit machine is a great tool and can improve some of the typical work they face. For cabinetmakers, it has much less value and for furniture makers even less. My 2¢.
Beat it to fit / Paint it to match
Dear Hammer,
You state: "I couldn't get clamping pressure on the joint and hoped the biscuits would help hold"Even without looking at the pic we all know that isn't going to work. Without clamping pressure, a joint like that will fail. The biscuits aren't going to save us from poor joinery. What I see in the pic is that there is no failure of the wood, anywhere along the joint. The entire joint failed. Don't blame the biscuits.
Beyond that, I tend to agree with your conclusions. Biscuits have their place. I use them quite frequently as I do not discriminate in my shop. I am all about what works. I'm not really sure how someone can rave about a spline, but look down on a biscuit. Snobbery methinks? Some of these folks who think that a biscuit add no strength to a joint, should try tearing one apart some day. They may reach a different conclusion.Best,John
Edited 1/22/2007 7:05 am ET by Jmartinsky
J,
The advantage that I can see in using a spline over a biscuit in reinforcing a miter joint are twofold: 1) the spline can be accurately sized to fit its recess, whereas a biscuit depends on the swelling of its compressed surface to fill the recess. 2) the recess for a spline can be sized to accomodate the size of the joint, whereas biscuit sizes are predetermined.
Regards,
Ray
Thank you for explaining what I was trying to, regarding a spline.
To John:
No snobbery here at all. A spline can be created to a maximum size, depending on the size of the joint, for maximum holding power. We all know (at least I hope we do) that the strongest 'glue' joint is one involving long grain to long grain. By being able to maximize the size of the spline, you are able to maximize the surface area of the long grain v/long grain glue bond. You can't do that with a biscuit.
Also, biscuits tend to be a sloppy fit. Some swell a little, and some swell alot. I've seen (in my own work years ago) biscuits swell so much on a slot that was perhaps cut inproperly, or just a slightly oversized biscuit (who knows?) out of the bunch, that swelled so much that I couldn't clamp the edges tightly together. The swelling of the biscuit can actually push the joint apart a little.
And lastly, I wasn't trying to say that biscuits don't work, and don't increase strength in a miter joint. They do. I was just stating that there are much better alternatives out there.
Jeff
I wonder if there are better quality biscuits out there that do a better job swelling than others? Much like name brand nails over generics in a pneumatic nailer.
In a way, I think that was what Festool was thinking when they designed the domino system. Time will tell.
Jeff
Jef,
You need to obtain a real bicuit joiner and some properly engineered biccies, my boy. These are available to us here in Europe, of course. I know that you will probably have to plunge slots with painted-tin joiners and buy cardboard biscuits from The Home Depot. :-) No wonder you don't like them.
Then there is the necessary skill in using the technology. It's a bit like learning to chisel properly.....at first you woggle the thing about and make three plunges instead of one, all in the wrong place. No wonder you get a sloppy hole and a recalcitrant biscuit. Cuh!
Lataxe
Sir Master Lataxe,
Are you screwzing with me???? I believe you are! I'm in no mood, as I'm still completely full of myself for actually picking the score of the Bears vs. Saints game yesterday in the pre-game festivities pool at the party I attended. You know, the real FOOTBALL. Not that silly little game where 8 guys stand around watching two guys kick the ball!
OK. Enuff funnin'. If ye' must biskit, then biskit away! Ye' won't narry find one of dem der dings' in any of the furniture I build!
Jethro, full of glory over Da Bears!
Jeff,
I have politely, and with great restraint, stayed out of this one. You are carrying the banner very well, thank you.
I feel that the entire plate joinery industry is an insidious effort by space aliens to cause slow liquifaction of human brains. Think about it. An otherwise logical, lucid, woodworker (usually male - dunno why that is), once infected with the dreaded desire to use biscuits, looses all critical thinking ability about what these little rotters actually do in a wood joint.
Even workers versed in proper long-grain to long-grain contact in a glue joint construction, workers steeped in the ability to make proper dovetail joints, M&T joints and all manner of good joinery become unable to appreciate the shoddiness these compressed wafers of beech and their oversize mortises wreak on mandkind.
*sob*
People like you and I shall remain strong against this destruction-from-within. Stand tall, brother. Never allow biscuits to sully our pristine joinery! We shall beat them on the beaches, in our towns and in our shops. Wherever the cross-hatched beasties raise their ugly football-shaped bodies, we shall beat them off!
Rich
Rich
OK. I'll carry the torch for a little while, but if they start throwing rocks, I'm gonna need a little help.
Given the fact that this site has many, many hobbyists who are self taught, or tv-taught, I can certainly see where all the hoopla about biscuits comes from. I wonder if Norm Abrams was paid by the biscuit jointer companies to include them in every single project he built in the New Yankee Workshop. Now, I have a tremendous amount of respect for Mr. Abrams. He almost single handedly kept the craft of woodworking in front of America for 2 decades, and is responsible for the desires in many, myself included, for wanting to work wood at a young age.
However, once skills become more developed past a beginning stage in this craft of ours, we would also hope that desire for more information comes with it. We all strive to make things better than before, and we buy tools with the hopes that they will make things easier for us to achieve our lofty goals. What gets lost, sometimes, is that it really falls back on us as individuals to discriminate between right and maybe. I won't say 'wrong', because I don't think it's wrong to use a biscuit. I just think that there is always a better way.
I think back 15 to 20 years ago, when I was using biscuits on every table top glue up, thinking that they were a necessity. I can't remember how many of those table tops made their way into the burn pile BECAUSE of the dang little biscuit. Either I put them too close to the edge, and exposed them with a router for some ogee edge detail, or they swelled up so big that 1/32" gaps were all over the place.
When I decided to pursue this craft to make some money, I also began looking for better sources of information on how to make myself a better woodworker. I began to realize that many of the things I took as woodworking gospel were just not so. Biscuit joinery was definately one of them. For aligning table tops, alls you need are a few cauls. They're re-usable over and over and over again, and you don't have to take the time to cut all those slots. For any type of end grain joint, or end grain to long grain (face frames) there are so many better and faster options available.
OK. I'm putting the torch down. You carry it from here. Lunch break is over, the table top I just glued up is most definately dry (without biscuits), and I have to go cut some dovetails for the jewelry box drawers I built my daughter for Christmas. Ahhhhh, just a little late with that one!
Jeff
Jeff,
I must be one of the few people in America who have never seen Norm in action. I have seen him grinning on "This Old House," seemingly offering sage advice and describing carpentry already done or about to be done. But I haven't had the pleasure of watching his shop techniques.
I was never aware of the so-called influence he had and his role in "keeping woodwroking alive in America," as I have heard admiringly from his supporters.
But then, I don't get out that much!
Rich
Jeff,
Aha! Hoist with your own confession of past biscuiting follies and handcacking!
" I can't remember how many of those table tops made their way into the burn pile BECAUSE of the dang little biscuit. Either I put them too close to the edge, and exposed them with a router for some ogee edge detail, or they swelled up so big that 1/32" gaps were all over the place".
It is a terrible thing to see a growed man that cannot do his biscuiting properly. Send me an air ticket (first class is my usual mode) and several hundred dollars. I will come to show you how to do the biscuiting with panache, vigour and a manly flourish, as you show your completed work to the neanderthals still messing with their first M&T whilst bandaging their fingers from misdirected or impatient chisel thrusts.
I will use the hundreds of dollars to purchase, just for you, a fine Lamello and various Superior European biscuits, such as Lamello's own (beech ones and fibre ones), Tanselli (multi-ply construction) and Trend (best German beech).
After an hour or two you will be telephoning Rich to convert him; and also that Ray.
***
Biscuits - sent down from The Gods with the nectar and ambrosia, for us all to enjoy and marvel at.
Lataxe, a missionary for modern joints; and your would-be saviour from a life of Primitive Woodworking.
Edited 1/22/2007 3:43 pm ET by Lataxe
Lataxe, Another victim of space aliens. Rich
Edited 1/22/2007 3:56 pm ET by Rich14
lataxe,always looking for your entertaining posts! dare not contribute as i fear keeping up with that wit!however, would love to see pix of your work.....no biscuits showing i trust.
M,
You could have a look at thread/post 30647.31. You will find there a biscuit-ridden object of mine. :-) It hasn't fallen to bits yet, despite the evil spells cast on it by the neanderthal biscuit-fearers.
Lataxe
biscuit ridden indeed!...beautiful work.evil "biscuit" spells be damned!
Lataxe, old top,
I'm simply filled with curiosity.
Where exactly do the biscuits ride? Do they have tiny little saddles, and do they store them with your other tack, afterwards? How many other items in your home have been biscuit ridden? Do your cowboy bikkies steer them about using bridle joints, and forstner bits?
If they were my biscuits, I'd get rid of 'em, before the furniture is riddled with holes from their eensy-weensy spurs. But maybe the little buckaroos aren't too rowdy. YEE-HAW!!! Bikkies waving miniscule sombreros, disappearing in a cloud of sawdust, into the sunset. It'd make a beautiful movie. Clint Eastwood might direct, and write some of the dialog: "Ya feeling punky, spalt?"
Ray Pine
Ray, old country cousin:
You have been using that 18th century finish with the datura root in it agin', incha? You know it gives you them surreal dreams.
Don't worry, the biscuits are safely encapsulated within the tallboy and cannot ride out with the red glint of revenge in their eyes to punish you for your badmouthing them.
Of course, the Mafell biscuit joiner is still a free machine and may be plotting even now to secrete itself in that Hog of yours, rearing up with a bitey blade at the critical moment as you are trying to make Dead Man's Curve in the rain. (I have heard the tale from Jan & Dean). Fear not; this scenario too is just another of your finish-sniffing hallucinations.
Lataxe, inducing AI in his WW machines.
Ray,
The advantage that I can see in using a biscuit over a spline in reinforcing a miter joint are twofold: 1) the biscuit is accurately sized to fit its recess, whereas a spline depends on the accuracy of the woodworker who made it to fill the recess (which must also havebeen carved well by that same woodworker. 2) the recess for a biscuit can be sized to accomodate the size of the joint, whereas spline will always show at the ends.
Then there these points:
* A biscuit slot can be made rapidly and accurately (assuming you have not got a ticky-tacky joiner and you understand how to use it).
* Biscuits swell and add mechanical grab to their glue grab whereas splines are glue grab only (and then only if you got the fit right).
* Biscuits are good, lovely, clever and nice; unlike splines, which are tatty bits of wood with no personality and smelly armpits.
Lataxe, a biscuiteer and president of the "Defend The Biscuit Agin' Them Prejoodissed Neanderthals" society.
Lataxe,
I knew, I just knew, that I shoulda kept my mouth shut, or my fingers off the keyboard. You say that spline strength depends on the skill of the maker for fit, then go on to admit that cutting a proper bikkie jt requires a certain amt of skill on the part of the user, and good equipment. I'd call that a draw. Having read about the various difficulties folks have had with the fit of the biscuits (either swelling too much, or not enough) it seems to me, that I'd prefer to have control over my own cock-ups, rather than letting an over-or-under compressed wafer do it for me.
Splines do not necessarily have to show at their ends. I've often cut half blind splines that only show on their inner ends, not on the show side of the work.
It seems to me that the recess for a biscuit must be of a size to accomodate the size of the biscuit, which may or may not be an optimal size for the joint. I'm just guessing here, but I've only seen biscuits offered in certain sizes. A spline can be as thin as a veneer, as short and as narrow as you wish it to be.
But I am not trying to dissuade anyone, least of all you, my dear sir, from using whatever tool or means of joinery he or she wants to use. You have my blessings to dodge back into your bolt-hole, and biscuit or pocket screw away, to your heart's content.
Best regards,
Ray, a ticky-tacky joiner; rather tatty, and with smelly armpits
Lataxe- are you possibly being ironic..Bicuits have as much place in the armamentarium of woodworkers as the rubbed joint, as does Nakashima's butterflies or the sophisticated "viking" joint.heck, on some of the jobsites I been on, the joint's don't need no reinforcing, Eric in CalgaryWe may not do good work, but we are slow!
Eric,
I always try to suit my mode of discourse to both the subject matter and to the quality of debate underway. :-)
Lataxe, waving his pig's bladder on a stick.
Master Lataxe
You should try dem der biscuits with some sausage gravy (pronounced saw-zeeetch) drizzled over the top. That makes em' swell up just right, and taste darn tootin', too!
Jethro
phew......nice to have somebody speak english around here :)
Was hast du gesagt? ..... No probleme.....!
LOL
Jeff
John
I sent a post to joinerswork, and part of it is addressed to you. I hope you read it.
Jeff
Dear Jeff,
Where is "joiunerswork"?John
Post 14
Jeff
I teach a beginning wood class and one of the projects is a small end table ( A Federal Table). We use a double biscuit joint for attaching aprons to the legs. Two biscuits side by side in 7/8 th thick wood really is pretty strong. Takes some setting up though. I'm kind of a traditionalist and was slow to join the biscuit crowd but I believe in the double bisuit. Of course, the advanced students do M&T. I'm beginning to feel the urge to learn about pocket screws........... somebody save me!!
Mid,
You HAVE been saved - you have seen the shining light of the biscuit!
For leg to table aprons, I have used 2 X S6 (the very large) biscuits. I glue them only in the centre (2/3rds their length) and have even pinned them with a 3mm hardwood barbecue stick. My reasoning is that the S6s are longer so help keep the leg/aron in line; the S6s are slightly deeper into the wood giving more glue area.
Of course, biscuits make excellent M&T joints as they fit so well and add mechanical grab to the glue grab.
Halleluyah!
Lataxe, Priest of The Biscuit
Lataxe, your Lamelloship,
" pinned them with a 3mm hardwood barbecue stick"
And did you marinate it in linseed oil, then grill the whole thing for 45 minutes over medium coals, afterward?
Ha, ha, hahaha
Your friend,
Ray, the biscuit-apostate
Ray,
I only burn them 18th century pieces with the scuttle legs, before they get me in the night time. You should hear them shriek and curse! Sometimes a sprite comes out and whirrs off into the sky (probably heading for your shop and a new home in a newly-made scuttle-legged thang).
Of course, if I had ever bought one of them modern scandi-blonde ill-proportioned sculpture cabinets, in a moment of weakness and fadism, I would burn that too. I should imagine it woud give up what little ghost it might have with nary a whimper.
Anyway, it's pointless trying to burn biscuited furniture because it is so strong that it resists fire, flood, hurricanes and even the wrath & malice of the neanderthals! Oh little biscuit, come to me. Come and sit upon my knee. You are nice and made of tree.
Lataxe, a Mafellow in fact.
Lataxe,
"Oh little biscuit, come to me. Come and sit upon my knee. You are nice and made of tree. "
Fie upon thee, biscuit, Fie! Begone thou, from off my thigh! Thou nasty thing, of compress`ed ply.
Yours in doggerel,
Ray
joinerswork & Lataxe,
And he (Lataxe) also makes comments about Neanderthals. And then he's taking on the attributes! Is he buttering up to Mr, Macou?
Lataxe: Get a proper dust pan and broom like we all have, and sweep the shavings and dispose of them! It's the old fashoined way, like the Neaander',....
Once you have mastered the fine art of woodworking with hand tools, you won't need all those fancy European machines and gladly contribute them to us, back here in the colonies.
I do admire the way you have adapted Lee Valleys Twin-Screw Vise with the York bench dog. Much appaluse! If you don't mind, I think I'll make one out of wood, as I can't find a local source.
Keep posting your inventive ways as they are inspiring. Don't let it go to your head though!
Cheers,
Bob @ Kidderville Acres
Kidderville, NH
A Woodworkers mind should be the sharpest tool in the shop!
Bob,
I am just "aping" the neanderthals for fun. :-) I always butter up His Excellency The Marccou, though, in the hope of future discounts, extra blades and so forth. Of course, he merely snickers at my crude flattery and puts his prices up.
There is a brush (two!) and a shovel in my shed. They are only there as an excuse for the ladywife to pop in and borrow - ie see what I'm up to. A nice sucker machine for the sawdust means not too much bending down, which in turn means not too many bruised bits from getting up under the corner of various cast-iron tables. (I am delicate, you know; and experienced in nursing bruises in apparently inexplicable locations of my personage).
This week I am cutting 451,678 DTs by hand (or it feels like it) for six drawers in a chest. It is taking forever, so I often glance longingly across the shed to where the Woodrat waits patienty for me to stop this neanderthal nonsense. So far, I have remained strong (ie pointlessly stubborn). In all events, you will not get my lovely machines by hoping for my Conversion. (I might sell the drum sander, though; one careful owner and cheap at the price).
As to the York thingy and the side dogs - these are not my invention. Like 99.9999% of other stuff I know, I got the idea from other humans (who got it from other humans, who.....ad infinitum). This makes me feel as humble as a bee, since I realise I am just one tiny information-processing mote in a Great Big Meme Colony.
Lataxe, a small speck at no particular place within a Vast Universe
When you make a joint where end grain meets long grain biscuits can add a lot to the strength, compared to the simple glued butt joint. That's because gluing end grain provides relatively little strength. The biscuits become "mini tenons" and can provide a servicable joint, if not the strongest possible. As a version of this biscuits in a miter can help keep that joint together--though moisture changes are still likely to want to open it up.
But whenever long grain to long grain, either edge to edge or edge to face occurs, the value of biscuits diminishes greatly. That's because the glue line, if the joint is well fitted, will be stronger than the wood itself. Interrupting the glue surface with slots for biscuits just makes the joint weaker. Perhaps alignment is helped with a biscuit every 18 or 24 inches apart, but I've always found it easier to adjust the alignment during the glue up without the biscuits. And, unless your biscuit joiner is of high quality, and your technique impeccable alignment can actually hurt alignment. Having biscuits "in the way" taking time and swelling with moisture from the glue can also make it more difficult to get the joint firmly clamped together. I see no value, and possible harm from using biscuits in such situations.
Fortunately, in most situations, strength from long grain to long grain joints is much greater than is usually needed, so little real harm is done by weakening the joint with biscuits. That's why its possible to find plenty of people who "use them all the time" without failure.
Steve expressed my thoughts exactly, if not more eloquently.The only thing I can add is poorly placed edge-to-edge placed biscuits can bring out some really colorful language when raising panels, trimming table tops, routing endgrain, and other odd instances. Or so some of my friends tell me.Butt & biscuit makes a fine utility quality joint.
Edited 1/22/2007 4:32 pm ET by byhammerandhand
Do these bicuits make my butt look big?
Do these bicuits make my butt look big?
No, but a couple might help close up that crack.
Ray
Ouch!Careful, Ray. remember, this is a family show. Political correctness counts. Matt may be emailing you soon!Rich
Rich,
Yeah, this should be on the fine plumbing forum.
Ray
Ray, saying that, "Everything's fine with me except for the crack in my backside," can get you into a spot of bother at immigration check-ins. The statement might result in you sitting quietly in a holding cell for a while with the sound of snapping vinyl on fingers in the background-- and a little practice at touching your toes soon to follow.
I don't know this from personal experience-- just a rumour I've heard. Slainte.Richard Jones Furniture
Your your big butt makes your butt look big. Sorry I could not resist.
miller,
If I'd used butt hinges, it'd swing out of the way, maybe?
Ray
No, all that fat makes your butt look big :-)P.S. Never answer this question from your wife that way.
John-
Look at it this way: adding biscuits is like praying. It probably helps, and there isn't any risk.
"Look at it this way: adding biscuits is like praying. It probably helps, and there isn't any risk."There's no risk as long as the time to cut the slots doesn't take away from prayer time! ;)Honestly, I rarely use biscuits anymore. For straight up edge joining, I don't see the need. My biscuit cutter gets alot of idle time these days. There is some argument that the biscuits can telegraph through the surface, but my motive for not doing them anymore is time.
Some years ago there was an article written in one of the woodworking rags though I don’t recall witch one. They had created all manor of joints both machine and hand cut versions of each then stressed the lot of them till failure occurred. The way the article was written was nice in the sense it showed each joints weak and strong points so you could employ them in an effective manor. That article might be the source of controversy of biscuit joints as it didn’t show them in a great light as far as strength goes.
When deciding on what joint to use in any situation the stress that joint will be under and the speed to create the joint is a concern to many shops. If for no other reason the speed of biscuits will keep them around
According to the reading that I've done, biscuits do improve butt joints mechanically.
But the real value (per the reading) is as an alignment-during-assembly tool. It can save time and aggravation while adding a series of face frames to case work (i.e., kitchen cabinets) in that it aligns the frames to the cabinets quickly and reliably while you glue/tack/clamp/screw the little dears to the cabinet.
Don't ask me to footnote my sources - I read a lot.
Mike D
Biscuits are splines. They are not structural.
On end grain joints, like miters... yes. Otherwise it is just an allignment tool, and not a very good one. My fingers and a dead blow work better.
Pardon my spelling,
Mike
Make sure that your next project is beyond your skill and requires tools you don't have. You won't regret it.
In every application that I can think of, there is a much stronger and better alternative to biscuits.
Some cabinet makers like them for aligning face frames without needing face nailing. In this application, I can see where you could choose biscuits as an option, although the same can be accomplished with pocket screws.
I have a biscuit jointer. The box hasn't been moved off the shelf in ages, and it goes unused. As your woodworking improves, I believe your uses for biscuits disappear.
Just my .02.
Jeff
There was a test done and published in FWW. Do a search of FWW and you will find it. The test was a comparison of the strength of mortise/tenon, biscuits and dove tails. Not the exact case you are describing, but it definetly gives an indication of the strength of the joint. This report does not determine if the strength of the joint is related to the biscuit themselves or the glue. In my opinion on the basis of structural mechanics, not wood working experience, I do not believe the biscuits provide a great deal of shear strength but are a great alingment device. The real workhorse is the glue, many of which have a strength of 3000psi. Trick with the glue is you have to read the fine print. The shear strengths stated usually outlines the clamping pressure required to achieve the full shear strength and it is quite high. I took a look at Titebonds website and it suggests clamping forces between 100 - 200psi. What does this mean, at 100psi clamping one would require a force of 75lbs/in for a 3/4" or 900lb/ft. Glulam manuf use a resorcinol adhesive that requires 100psi to achieve the correct bond, and the clamping setups to build one are very impresive to say the least. An advantage to epoxy type adhesives is many do not require the same pressure to achieve their full strength.
John and all
For those who shun the biscuit, and in my opinion they definitely have their place in the wood shop, here is a joint I use to join two panels together. It eliminates the biscuit and the spline, yet still increases the gluing surface and also aligns the joint. JL
Methinks the short version of all this is that long grain to long grain, biscuits are probably a waste of time. End grain to either end grain or long grain is another matter. If the biscuits are a tight fit, there is some structural integrity added because they act as small tenons. If you feel the need to use biscuits in joints using wood of 7/8" thickness or less, let the joint dry/cure for at least a couple days before sanding for finishing so the moisture will migrate from the wood and return to a uniform thickness. (Don't ask me how I know this!)
Good luck
In my opinion, one place where biscuits are indispensable is for joining the mitered joints of door and window casing. With biscuits, the two pieces will remain even and flat. Without biscuits, the pieces will often twist and be misaligned.
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