Rules for Mortise and Tenon Joinery

I wanna make sure you understand the geometry of mortise and tenon joinery so that when you're putting together projects similar to this stool and using mortise and tenon joints, everything goes the way you expect it to go. So let's talk about, first, the scale of the tenon relative to the size of the rail. That's gonna lead, then, to the size of the mortise. So here's the deal. Starting with your rail material, you'll divide that by three, you'll divide the thickness of the rail material by three, that leads to the size of our tenon.

So, for example, in 3/4 inch stock we wanna put a quarter inch tenon which then, of course, also means a quarter inch mortise. Now, once we know that, the next thing we wanna know is how long the tenon should be to have adequate penetration into the leg. The rule there is to take the thickness of the tenon times two. Now, it's okay to go beyond that if you wanna make it longer, that's all good, but you wouldn't wanna be less than that. So again, using our example here, 3/4 inch material resulted in a 1/4 inch thick tenon times two results in a half inch long tenon.

Now, on your leg, when you go to cut the mortise that this is gonna fit into, we're gonna make that mortise just a little bit oversized. We're gonna cut that about a sixteenth of an inch too deep. That results in little spot for glue at the bottom so the glue doesn't hold you up when you're trying to put everything together. Now, looking at the tenon this way, we've taken a shoulder off here. We do also wanna take off a shoulder here and a shoulder here.

And the real deal there is that if we made the mortise exactly as long as the rail is wide and made just a little bit of a mistake, the rail got a little too narrow or the mortise got a little bit too long, when we put everything together, the mortise is gonna show above or below the width of the rail. So that's why we wanna use these shoulders here, that's gonna mask the mortise. Generally what I do, if I've got a quarter inch shoulder in this direction, I stick with that height setting and make it a quarter inch shoulder here and a quarter inch shoulder here. That way, for instance, on the table saw with one setup, you're ready to go for all four of those surfaces. Now let's talk about some logic here.

We talked about taking the thickness of the rail divided by three to get the size of the tenon. You wanna use some common sense and round that to the nearest size cutter you have. So if this is 13/16, don't just divide by three and try to make a tenon that's whatever funny size that would be because you probably don't have a cutter that would make a mortise that size. So it's thickness divided by three, round it to the nearest size cutter that you've got. That leads to a sequence of events.

You're always gonna make the mortise first, 'cause that's cutting a hole which is gonna be located and designated by some cutter that you have. Then, we're gonna go back and make a tenon to fit it. So sequence of events is do the math based on the thickness of the rail. Once you know how big the tenon has to be, then cut the mortise to that size, then come back to the rails and cut the tenon to fit. Follow those rules and you'll make sure that you've got good strong mortise and tenon joints so when you make projects like this, they're gonna last a really long time.