In Chris Schwarz Blog, Joinery

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Have you ever wondered why there are specific rules for the sizes of mortise-and-tenon joints? Did you know there are rules? If you consult the 19th and early 20th century texts, they state that tenons should be one-third the thickness of your stock. And that the tenons should be five times as long as they are wide.

So if you were cutting a tenon on 3/4″ stock, you would make it 1/4″ thick and 1-1/4″ long. Is this arbitrary? And why do more modern texts call for tenons that are one-half the thickness of the stock? Recently I’ve been cutting a lot of mortises by hand to test some mortise chisels. And as I get more comfortable with the chisels, the old rules for this old joint seem to make sense.

For example, when I work in 3/4″ stock (especially anything slightly fragile, such as cherry or walnut) a 3/8″ mortise chisel will absolutely destroy my work. It’s just too much steel plunging into the wood, and the 3/16″ walls of the mortise are simply too fragile. When I step down to a 5/16″ chisel, things become very manageable in oak and ash, for example, and workable in cherry, walnut and mahogany). And when I step down once more to a Ã?¼” chisel, I pick up serious speed in the harder woods and more control in the softer ones.

So what about that modern rule that has us all cutting 3/8″ mortises in 3/4″ stock? I’m starting to think that’s a machine perspective. Boring a 3/8″ mortise with a hollow-chisel mortiser is a simple thing in 3/4″ stock. In fact, I find that chip clearance with a 3/8″ hollow mortise bit is more efficient than with the 1/4″ bit.

And then there’s the issue of the tenon’s length. Why do we make tenons five times as long as they are wide? Was this to provide more gluing surface with less-reliable hide glues? Perhaps. Was it to ensure more of an interference fit between the tenon and mortise to beef up a sloppy hand-cut joint? Sounds good to me.

I think it also has something to do with drawboring (surprise, surprise). Here’s my highly questionable theory: You have to locate the drawbore hole a certain distance in from the edge of the mortise. I like 3/8″. Any closer and you risk cracking your stile when you drive in your wooden peg. And if your tenon is any shorter than 1-1/4″, then you risk blowing out the grain in the tenon, ruining your mechanical fit. There are probably other valid reasons for the rules that govern this joint. Have a theory? Let me know.

Christopher Schwarz


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Showing 5 comments
  • Jeff Ranck

    A discussion of this quote occurred on the WoodNet forums. A point of clarification (terminology). I’m thinking that what you mean is the length should be five times the thickness of the tennon rather than width. Is that correct?

  • Christopher Schwarz

    Dave,

    I think 3/8" or 1/2" will work quite well. I used 3/8" x 6"-long pegs to pin the legs to the top of the Roubo bench and it is beyond stout. In fact, I think it’s on this larger scale that drawboring really comes into its own. There is much less risk involved becuase there’s simply more meat in the joint.

    The most difficult drawboring jobs are the small ones.

    My only concern would be style. A 1/2" peg will be quite obvious to the eye. If the bed is of a style where that’s appropriate, then go forth.

    One more detail: I’d consider driving the pegs in without glue in the joint. That way you could disassemble the bed by boring out the pegs if need be.

    Chris

  • dave

    Hi Christopher,

    When drawboring, I know that you should offset the hole in the tenon by 1/16 to 3/32nds. But, I was wondering, how do you adjust that when using different sized tenons? I was thinking about this last night, guessing that the larger the tenon, the smaller the offset.

    I’m building a bed for my wife and I and I’d like to drawbore the head and footboards to the legs. The legs are about 3-1/4” square and the head/footboard is 1-5/8” thick. I was thinking of using a ½” or ¾” peg for drawboring – larger maybe being more impressive. I’ve spent a lot of time building the members – they’re hard maple, smoothed using my LV low angle smoother 🙂 – and I’d just about die if I blew out a mortise shoulder or split the tenon.

    Thanks,
    Dave

  • Christopher Schwarz

    Dave,

    There are two good books for woodworkers on the Golden Section: "The Old Way of Seeing" and "Measure Twice, Cut Once" by Jim Tolpin. Tolpin’s book is about (in part) applying these mathematical principals to everyday work.

    The "Old Way of Seeing," one of my favorite books ever, is more of a "use the Force Luke" book, and it insists that if we design with our eyes (and not our rulers) then we will naturally create objects that fall into the Golden Ratio (1:1.618).

    Your comment about balance is well-founded. That is the exact word used by Moxon when he describes how to proportion tenons. And it has as much to do with the material itself (density etc.) as it does with its size.

    Thanks for the comment!

    Chris

  • Dave Brown

    The old rules seem to make more sense than the new ones. They also more aptly to hand tools, which is good for us. I think that the old rules regarding mortise and tenon construction had something to do with proportions and balance. Old world craftsman were more in tune with the artistic side of their craft than alot of us today. Does anyone have any information on the Golden Mean or Golden Proportions and its ratios?

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