Why two tenons

[Dennis McDonaugh]

Boy, its been a long time since I posted--its just too hot to go out in the shop from May to September here in south Texas. I've been reading the board, but not posting much.

Anyway my next project is a highboy and I'm going to make legs similar to these. I'll be using M&T joints to attach the back and sides to the legs. I've seen people break the long tenon into two or three smaller tenons and the reason they give is wood movement. My question is why would movement be less with two or more tenons than one. Isn't the movement across the board going to be consistent whether there is one or two tenons?

[Jim Becker]

The wood movement isn't "less" overall, but it's easier to deal with with two smaller tenons than it would be with one long one. For example you could glue one and "pin" the other to allow it to float. Ya, you can do that with a long tenon, but it's easier to effect with two.

[lou sansone]

that is a good question

from my study of case pieces of the 18th century, many had two or three separate tenons on the sides and the back of the base of the highboy or the back of the lowboy. In the case of authentic 18th century pieces all the tenons were pegged that I have seen photos of or measured drawings made from museum examples ( understanding the modern idea of pegging one and letting the other float as is sometimes done in a headboard ) . One possibility was that it was an simply easier to chop 2 or 3 short mortices rather than one long one. But I am not an expert by any means and I also would be interested in what other folks feel it the reason for the tradition of the narrow tenons on case pieces
Lou

[Jay Knepper]

I've had the same question. Several literature references imply that dividing the tenon in itself helps to deal with wood movement (with no mention of whether or not all divisions of the tenon are glued.) This makes no sense to me.

Usually divided tenons are shown as being double. It would seem that dividing long tenons into three sections and gluing only the center one would be best practice. Gluing in the center minimizes the lever arm of forces in the plane of the tenons. One exception is breadboard ends on table tops. In breadboard ends gluing the center only and pegging the end tenons (or the ends of a single tenon) into slots is commonly described.

Lacking any clear guidance, I don't glue more than ~3 inches of tenon, I glue the center, divided or not, and peg the ends into slots if there are concerns about strength. But I've been wrong before.

I wonder if someone has not done a survey of old furniture to try to get a handle on what has worked in practice. Lou, while each section of the pieces you mentioned were pegged, is there any way of telling how much of the longer tenons were glued as well?

[Mark Singer]

Dividing a long tenon in two has certain benefits. The long thin tenon as one monolithic element has a tendency to twist. This is true because the thin section will allow the nature of the wood to manifest.....if it is not quartered and has eccentric rings it will develop twist. When divided into to it breaks the effective width and reduces the internal stress....therefore less twist. It is very much like wide plank flooring which tends to cup....if the boards are 4" wide instead of 8" it is going to cup less.... The twist in a joint will offset the surfaces and create other problems. Haunching the ends reduces twist since the effective width is reduced. Two tenons side by side is used to increase the gluing area.

As a side note if you use two loose tenons you have the ability of selecting wood that is less likely to twist since you can chose the grain pattern.

[tod evans]

dennis, i think the benefit or reason of using two tennons is twofold, first as lou stated the two smaller mortises take less time to chop. and as mark says there is less chance of twist occuring when the tennon is broken for a couple of reasons, first as mark said because the grain is severed but more importantly the mortised section will remain more rigid simply because its integrity is only broken in a couple spots instead of full length...02 tod

[Mike Cutler]

Dennis.

Not much to add except that I've always been led to believe that the purpose of two tenons instead of one was to protect the integerity of the mortised member. One really long tenon could blow out the sides of the mortise if it started to move ( twist, or cup).
The gluing stategy for the divided tenon is going to be different depending on the application. A wide panel glueup is going to need more "float". Two pieces meeting at a right angle, where strength is needed, may be glued differently. The entire joint may only be 5-10 inches wide we'll say. In this application I have pinned, and glued both tenons in their mortise. There isn't enough wood movement in this type of joint. This of course is dependent on the species of wood. Some woods move a lot more than others. Additionally quartersawn moves less than facesawn. The tangenital grain in facesawn has a lot more movement, when compared to quartersawn. Grain orientation can be just as important as joint selection in some of those period type pieces for overall strength and longevity.

My current project on the bench has divided through tenons, but they are for visual asthetics primarily. The entire width of the tenon area is < 3". Wood movement isn't going to be an issue.

[Jesse Cloud]

Mark, Lou, and Tod have 'nailed' it. Here in NM we go from single digit humidity during good weather to saturation during our monsoon season, so I worry about wood movement a lot. I break any tenon longer than three inches into 2, leaving one floating with a little extra room if I'm working in the dry season.

Its not my favorite tool, but the Leigh FMT jig really shines on double tenons - two are almost as easy as one. Same setup, just set two horizontal stops on the table, cut your first tenon, unlock the table, shift to the second stop, relock and cut the next one.

[Dennis McDonaugh]

Thanks guys, I guess twist is a type of wood movement I didn't think about since I was focused on shrinking and expansion of the joint. Two it is.