The Unpredictable Backstool

A coarse phone shot of the chair before I trimmed the legs to final length. It's sittable, but needs a lot of clean-up before paint.

A coarse phone shot of the chair before I trimmed the legs to final length. It’s sittable, but needs a lot of clean-up before paint.

The three-legged form of backstool is ideal for uneven or dirt floors, though it looks wrong at first to modern eyes, like a Zap Xebra three-wheeled car. Though we all know in our heads that a three-legged stool is stable, adding a backrest to it throws our eyes off.

Even Victor Chinnery, the dean of English furniture, wrote the following warning in “Oak Furniture: The British Tradition” (Antique Collectors Club).

“Three feet will stand with greater stability on an irregular surface, but it nevertheless takes a certain amount of skill to sit comfortably in such a chair, since it is easily overbalanced.”

Judging from the number of extant three-legged backstools, that statement seemed like it was written with the eyes, not the buttocks. But the only way to test the statement was to build a three-legged critter and sit in it after a few beers. So I did.

As I designed this backstool, I followed the geometry I found in other three-legged backstools and chairs – usually the back leg rakes backward significantly. So I was careful to replicate that feature when I made models of three-legged stools before building one.

As my backstool came together I sat on it at every stage in construction. At first I expected to be tossed to the floor. That didn’t happen. And when I had my first formal sit-down in the completed backstool, here’s what I felt: stable.

My front legs were planted over the front legs of the backstool. My tailbone was on top of the back leg. I leaned back and my head hovered over the footprint of the rear leg. I cautiously creeped my buttocks left. Then right. I reached for my fourth beer.

And… nothing.

How does the backstool get its reputation as tippy as a drunken uncle? Part of the instability is an optical illusion, but part of it is real. It just has nothing to do with sitting on the chair.

We use chairs and stools for more than sitting. If you stand or kneel on this seat and the pressure is outside the triangle created by the feet, you’ll get a rude surprise. Or stand behind the chair and lean on the crest rail. If you lean on its center then nothing happens. But if you lean on one end of the crest rail, you might just bite the floor.

If you aren’t sold on the idea of a three-legged chair, that’s OK. It’s simple work to make this backstool with four legs instead of three. But consider this: If you do have the guts to make the three-legged version you’ll never have to yell at your kids for tipping backward in their backstool.

— Christopher Schwarz

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31 Responses to The Unpredictable Backstool

  1. I’m very impressed. It was my assumption that the reason a typical three legged stool works is because there is no back. You’re forced to sit up straight. I would have guessed that sitting back would have quickly thrown off this balance. While, it doesn’t appeal to me aesthetically, it is a nice little experiment.

  2. lburgess9515 says:

    Just for the sake of argument, wouldn’t the 3 legger be more stable with the 2 legs under your rear because that is where most of the weight load is?

    • mcdara says:

      I have to say, I wondered the same thing. If you had two legs in the back you could lean/tip back on the two legs, but I wonder if you would get the stability that it seems to have, per Chris’s description, in this configuration. In looking at the seat shape, I would think, if you changed it to ‘one in the front’ you might want to shape the seat differently, so as not to have as much of the seat “outside the triangle of the feet”.

  3. Ryan Starkey says:

    Chris,
    Lovely! It looks “wrong”, I will definitely be building one soon. I understand the tapered tenon being tightened by the weight of the person, did you feel exact matching the taper of the mortise (via lathe) was that difficult? I’ve had good results on stools with air dried seat/kiln dried legs and wedged, but those were straight round tenons.

    • I use the standard tapered reamer and the 5/8″ tapered tenon cutter – both by Veritas. They are inexpensive and work extremely well. You don’t need a lathe. You can shape the tenon close to size with a drawknife and then finish the taper with the tenon cutter. Easy.

      Some early pieces used cylindrical joints that were back-wedged. I prefer conical joints that are back-wedged.

  4. wb8nbs says:

    What brand was that beer? Doesn’t sound very effective.

  5. spokeshave27 says:

    Like the Reliant Robin, the trike and the three legged stool – they can be stable in the hands of the skilled user and within the laws of gravity and sans alcohol. But after a few beers I am inclined to lean/rock back onto the rear two legs and still remain seated so long as the centre of gravity, as you say, remains within the triangle created by the legs. However, in your case the back has only one leg… great for spinning as the inebriated sitter swivels to the floor. This design lends itself to sitting backwards – leaning over the crest rail.

    However I love the look of it!!

    • jonathanszczepanski says:

      Reliant Robin you say?

      • jenohdit says:

        Chris has the 3rd leg in the back.
        File this under cars of necessity.

      • carpenterman says:

        ah, Top Gear, my favorite woodworking show.
        I guess these videos answer the earlier question about stability with the ‘two legs’ in the rear theory. I like the ‘Morgan’ better.

    • Daniel Roy says:

      These thought experiments, though more sophisticated than Einstein’s, leave a bit to be desired. So, I’ll add mine. I would think that if Chris sits backwards in the chair it would feel maybe a bit unstable if he reached and lean out to the left or right because the back of his thighs would put pressure on the edge of the seat outside the triangle. This edge is furthest form the triangle for this shape of seat. But, the reason and chair feels stable when sitting in in backwards is that the plane of chair seats are not parallel to the floor. They slope back. Even Chris’s. This lifts the back of the thigh off the plane of the seat and a three legged chair becomes five legged (Two human legs added). Weight is transfered to your feet. Your body (brain) naturally adds stability utilizing your feet without you beings aware and it ‘feels’ very stable. Until you have to many beers and you brain can no longer help yourself.

  6. mokusakusensei says:

    The Tage Frid stool has three legs. “Nothing to see here…lets move along”

  7. Wesley Beal says:

    Chris, the sacrifices you make in the service of woodworking continues to impress me.

  8. Looks like a three-legged dog. We called them multiplication dogs. Put down three, carry the one.

  9. toolnut says:

    Bet I could fall off it.

  10. bsrlee says:

    Can’t tip back? Ha! You obviously don’t know my friends – or me for that matter. Again, its a matter of the user’s legs providing the ‘extra’ balance points, with the seat-of-the-pants being the connecting mechanism.

  11. Brian Eve says:

    Most of the photos I’ve seen of old three legged chairs like this have the legs configured exactly the same, with the single leg in the back. There must be a reason.

    At first thought, it might seem that the single leg in front would lead to more stability since we spend so much time leaning back in chairs. But, perhaps a few hundred years ago it was different: Sitting while weaving, knitting, putting logs on the fire, even reaching across the table for salt would be better served on a chair such as this, as the leaning to the front and sides are required.

    Or, is that the beer talking?

  12. A three legged chair is far more stable (predictable even) with two legs at the back. I can’t imagine how the issue is even up for debate.
    Chris, find the three legged version Arne Jacobsen’s Ant chair and sit in it. It’s much more stable than any seat (backed or not) with two legs in front.
    http://www.fritzhansen.com/en/ant-chair-3100-3-legs-stackable

    • Daniel Roy says:

      Thanks, we can all go home now.

      • You’re welcome. I’m glad I could could bring this to an end. I gather you’ve sat in both types and agree that there is far more stability in a chair with two legs at the rear than one with a single rear leg.
        When one leans back, one doesn’t have the additional stability provided by one’s legs that one has when leaning forward. Leaning across the table for salt(?!) hundreds of years ago would, I assume, still mean that one’s legs are able to provide additional stability. I’m glad you agree that one isn’t able to rely on one’s legs in the same way when leaning back.

  13. rushbycraig says:

    Great chair! I love the simplicity of it.

  14. spokeshave27 says:

    I tend to take photos of things that I might want to make. I have been going through hundreds of images this week and came across this: http://sawdustandwoodchips.com/wp-content/uploads/2015/01/2015-01-29_10-31-49.jpg

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