# Mortise and tenon joint proportions for 3 wooden beams corner

I'm designing a cabinet with drawers and I would like to use 60mm (roughly 2 and 3/8") wide beams for the frame. The frame with one side panel visible looks like that:

I would like to use mortise and tenon joints as they seem like a simplest solution and are relatively easy to cut. The problem is I'm not sure how to size them. You can find multiple rules for such joints in the internet, one of them being (this is from Christopher Schwarz):

• tenon's thickness should be one third the thickness of the stock being mortised
• tenon's length should be 5 times its thickness
• tenon's width should be half the width of the stock you're cutting it in

This doesn't make a lot of sense in my situation, because one third of 60mm is 20mm and 5 times that is 100mm, which is longer then the beam width. And even if the beam was wider there won't be as much place, because there will be 2 horizontal beams connected to the vertical beam. I found three simple ways to do the joint in such situation:

1) Making ends of both tenons slanted:

2) Cutting a through dado in one of the tenons:

3) A variation of the second joint:

Would any of these joints be suitable for the situation? What would be best proportions then? I can't find any good source of information on joining beams in such a way. If any other type of joint (dovetail?) is more suitable for such situation, I'd love to hear about it too.

• I would avoid option 3 at all cost. Unless you cut 2 half-mortises, 1 to full depth, the other to 1/2 depth, you're going to have a significant portion of each mortise unfilled with anything but dripping glue. Apr 9, 2016 at 15:36
• Good answers below, but if I was to opt for something simple and good, it'd be #1 above. Alternately, you could do one as a double tenon and the other as a single that slips between the double. Apr 9, 2016 at 16:55

This doesn't make a lot of sense in my situation, because one third of 60mm is 20mm and 5 times that is 100mm, which is longer then the beam width.

I think it's important to remember that these, as in all similar general guidelines about how to size a joint, are just rules of thumb and no rule of thumb needs to be followed slavishly. That's what makes it a rule of thumb, and not a Rule :-)

It's also worth pointing out that most traditionally joinery in cabinetry is actually over-engineered, resulting in individual joints that are much stronger than truly needed. This is no bad thing for longevity, but where you're not making something to last for a century or longer you can make certain concessions and not suffer any penalty.

With regard to mortise-and-tenons joints specifically, these are inherently strong, and you can definitely use very different proportions for the tenon and achieve a strong joint — just look at stub tenons for example, the length can be as little as 1/5 that suggested! Just to clarify, this isn't to be preferred, but it does show the kind of thing you can get away with.

Also worth comparing those proportions to dominoes and even biscuits, which are a simulation of M&T joinery without the need to actually cut tenons. These are in the main shorter (sometimes much shorter) and often thinner than a tenon or a pair of tenons would be.

I found three simple ways to do the joint in such situation:

Of the three options shown I would go with #1 in the sense of having the mitred ends, from my reading that would be by far the most common solution. However, I wouldn't necessarily use a small, square tenon. It would be better to have the tenon like this:

[Source: The Joint Book by Terrie Noll]

But I think most common traditionally (or at least most commonly recommended in texts) would be to use haunched tenons:

Obviously these are much more effort to cut and like I refer to above are likely to be an example of over-engineering. I include them here more for completeness rather than as an actual recommendation.

If any other type of joint (dovetail?) is more suitable for such situation, I'd love to hear about it too.

You could in theory at least use a dovetailed housing joint (short dovetailed tenons at the end of each rail, with a matching housing or dado in the corner post). I've seen it done once or twice but I couldn't locate an image. Again this would be more work to create than the simple mitred tenon shown first and probably provides no real benefit at the end of the day.

• +1 for "slavishly." Aside from reading Neal Stephenson books, I don't often need to google for vocabulary assistance. Jan 19, 2017 at 5:52

I would try to keep the geometry of the tenons as simple as possible. The important thing is to maximize the contact surface between the side grain surfaces in the glued joint. Since your making a cabinet base you need not totally conceal the connection (I assume you have a top counter surface over the base).

Each tenon is half the height of the beam depth. The top one will be exposed at top of the column member and the bottom tenon will be fully embedded. They will be glued to each other and to the column inside the mortise. The width of the tenons is not critical other than you want to create as much contact surface as possible and the vertical side surfaces of the tenons should provide plenty. I would keep the surfaces of the tenon, both sides and ends, at least 1/2" from the exterior face of the columns to insure adequate strength in the column member.

• What is this called? Does it have a name. It's pretty cool. The only thing is the inner corner of the tenon would be weak so this is not a configuration to use if you plan on disassembling the unit.
– Matt
Apr 9, 2016 at 14:28
• No name that I know of. As to strength, the inside corner is at least 1/2" square in cross section extending up 1 3/16", so I am not to concerned about it failing.
– Ashlar
Apr 9, 2016 at 18:24
• Handling the corner mortise and tenon joint has always kind of bugged me about how to handle the potential interference of the tenons - not that I have made any. I have not seen this solution before and suddenly the corner m&t no longer bugs me +1 Apr 9, 2016 at 21:11

Given what you are designed you might be over thinking this. Each of those tenon designs would suffice. The last two would end you with a nice outward appearance. If aesthetics is important to you then I would consider the latter two. The last one you have with lapped tenons would be the most difficult to cut but still feasible.

Why I don't think this matters is that you are building a cabinet with drawers. The should not be an excessive about of weight on this unit that would warrant anything other than the mitered tenons you show. Assuming you are going to put a top on this that would also distribute any weight present.

If I am incorrect or weight is a concern that I would also consider Ashlar's answer as it is very simple and would mitigate that better.