Interlocking joints; post & rung chairs

David Douyard & I live within about a 2 1/2 hour drive from each other, yet we’ve only ever met in Australia. But we’ve traded notes & phone calls here & there. About chairs. Yesterday he wrote with a question about the interlocking joints on Jennie Alexander’s chairs. Not something I’ve gone into detail on before, so a chance to think some more about chairmaking and JA, now four years since her death in July 2018.

side rung locking a front or rear rung in place

Back in the 1978 edition of Make a Chair from a Tree, Alexander built the front and rear sections of the chair first, then bored for the sides. She used the interlocking joints (photo above) to pin the front (or rear) rungs in place with the side rungs. This photo is from those days – the mortise is bored with a forstner bit and the tenons have shoulders – it might even be turned. Looks like all hickory.

I have an early JA chair here, made about 1973 or 1974 before she used interlocking rungs. It’s turned, all hickory. Shouldered tenons bored on centerlines, not on tangents. A beastly uncomfortable thing, but an important (to me, anyway) chair.

early JA chair

JA did not cook up the interlocking joints She learned the technique from studying old chairs in museum collections, disassembled ones were the best. Before she learned photography, she’d commission black & white shots from museums she’d visited with Charles Hummel. You can see in the photo below that both mortises are shifted above & below the tangent layout line.

disassembled post & rung chair joints

This next one is a great photo showing the relationship to all these parts. The post with the mortises in it has been turned around to show us the mortises. Note the notch on top of the tenon at the bottom right in the photo. And you can clearly see the layout struck on the post, Great stuff.

interlocked mortise & tenon joints

Alexander drew the joint a million times to better understand the mechanics and to tell whoever would listen. And Alexander was a tinker-er. Locking the front and rear rungs in place was not good enough for her. She decided, very early on, that the main stress on a chair was fore & aft. So why not assemble the sides first and lock those in place? This sketch has the chamfer at the end of the tenon, flats on the sides and even the circumferential notch (later dumped by JA, Drew, etc). But clearly labels the side rung as the “subservient” tenon in this case.

That’s where she was when she & Drew Langsner met in the late 1970s. Drew helped figure out how to go about assembling the sides first. From then on all the JA chairs were built sides-first. Not at all intuitive. But it works.

And one of JA’s favorite parts was making test joints and cutting them open. Both to see the result and to capture the perfect photo of it. We shot hundreds of this sort of thing, both for these joints and the drawbored mortise and tenons we used in joinery. This one you can tell is a later-period example from the top of the blog post. All oak now, white oak at that (maybe it’s a red oak post). No shoulder on the tenon – all shaved. I’m not sure how that mortise was bored – there’s no lead screw of any kind.

later JA cross section

I imagine eventually this one would be rejected – the mortise isn’t deep enough in the post. She preferred a very thin post, 1 1/4″ or so. Less sometimes. And a 1″ deep mortise. That’s pushing the limits of the material. It can get pretty frightening at times. Note the split in the post where the top tenon reaches the bottom of the mortise.

detail of above

Is this technique necessary? No, not at all. Millions of post & rung chairs have been made without interlocking rungs. I still do it – I like the history of it and it’s fun. But it means nothing. I still flatten the sides of the tenons too, and Drew told me he stopped doing that over 30 years ago!

But I did dump the circumferential notch.

the circumferential notch

It’s simple to do if you’re turning a chair, but if you’re shaving it the notch is a pain. When the first book came out, there I was with a Stanley utility knife carving this stupid notch around the top & bottom of each tenon. Eventually JA decided that the most important surfaces on the tenons were the top and bottom and the notch removed material from them. So out it went. Some makers of turned chairs still use it. I bet it’s fun. JA’s note in the 1978 text says “some chairmakers used more than one notch” – how about three??

three notches

The interlocking joints made it into the new edition of the book. The notch did not…

10 thoughts on “Interlocking joints; post & rung chairs

  1. The problem with the interlocking joint is that it becomes almost impossible to take the chair frame apart for a repair. Or in case a joint gets loose.
    Still, I do it.
    as Peter mentions, I dropped the circumferential ring and slab sides long ago.

  2. I saw the notches on chairs I reglued in the 1960s and never understood what it was for. More glue? Releasing excess glue? The post shrinks into it? I also aw chairs with such pronounced notches, you’d call them buttons or toggles or something.

      • The flattened sides of the tenon reduce the chance of the post splitting as it shrinks around the tenon/rung. Would probably be helpful if the posts are very green at assembly, which maybe some antique/traditional chairs were. The notch is reported to also be a way to lock the joint together – post is supposed to emboss into the notch. That might happen when the dry tenon absorbs moisture from the post and swells…that’s some of the reasoning anyway. again, I think you’d need wetter posts than we use these days. And I might also argue that species matters. Many traditional chairs used posts of woods like maple, with rungs like ash or hickory. JA switched the posts to oak. Rives more predictably than maple…

  3. Thanks for the detailed explanations! The interlocking joint is hard to wrap around (the tangential layout line explains it well). Also I always wondered if you were supposed to lock the sides or the front For very small parts I found that the notch works as advertised in m. Langsner’s book. I microwave black walnut 3/8 th inch diam. tenons before inserting them in air-dried thuya legs (for example making a toy horse), the notch being centered inside the leg, and once equilibrium has been reached the leg can swivel around the tenon without coming off. Do not overdo the notch however because lateral forces could break them at that weaker point..

  4. While working with JA on the 3rd edition, the question came up – which was the most important feature holding the chair together – the slightly oversized superdry rung tenons going into the moist posts, or the front and rear rungs interlocking the side rungs? JA thought it was the interlocking rungs.

  5. yes, 26 years ago JA switched to oak, easy to get and store in the backyard water tub. As the “commander” brought its intentions upon the old oak, the blue acidic stains were ever present in the aromatic red log. Peter, I so respect your acknowledgment of JA as, JA.

  6. This is excellent information, thank you for sharing.

    I love how the rungs in the later cross section have about half the growth rings compared to the earlier one—surely another important improvement.

    I wonder if the last 1/8″ or so of that mortise could’ve been finished with a second, identical bit to which both the lead point and spurs have been removed. A bit fussy, but in a post as thin as 1-1/4″ or less I’m sure every bit of fiber that’s left intact would count for something.

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