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Inline vs Angle Tenons for Compound Mortise and Tenon Joints

12 comments
12 years ago
Joinery
Angled Mortise and Tenon

Typical 10 degree splay in a mortise and tenon joint

While more obscure than the age old pins or tails first argument, another proverbial hornet’s nest discussion is whether to cut angled tenons or inline tenons when making angled or compound mortise and tenon joints. I’m no chair maker (the most prevalent use of these joints) but I spent the weekend cutting compound tenons while filming a Hand Tool School lesson. This was the first time I did a lot of these joints consecutively. In the past I have cut maybe two of these while making a project. While a good learning experience, it is nothing compared to making 30 of these joints in a single shop session. That pretty much sums up my weekend and my thrill-a-minute lifestyle :). So flush with this experience I have my own conclusions.  I fall in the inline tenon camp.

Let me ‘splain…

Angled vs inline tenonsThe argument is that because the inline tenon with angled mortise is harder to cut, the angled tenon with square mortise is a good alternative that doesn’t greatly sacrifice strength of the joint. The opposite view is that the reduction in strength due to grain run out in the tenon is something that cannot be overlooked in a high stress joint like the back leg of a chair. Pictured are two tenons that make up joints with a 10 degree splay. The angled tenon on the right makes the deviation away from the grain while the inline tenon on the right stays parallel to the grain. Obviously the more the tenon can parallel the grain the stronger it will be so the angled tenon technically is weaker. However look at the splay on this joint which is pretty typical of the trapezoidal seats of classic chairs. It really doesn’t deviate much from the grain at all and I can’t imagine it would sacrifice much strength.

So why not use an angled tenon since it is easier to cut?

chopping an angle mortiseFirst, laying out this joint is more difficult. Since the end of the shoulder is angled and the tenon is drawn perpendicular to that angle, a typical mortise gauge can’t be used. Instead I found laying them out with my bevel gauge referenced against the end grain to be the best method though it was not very easy and allowed for a fair amount of error to creep into the joint from one to another. This can be a problem when 2 to 4 of these need to be cut when making a seat frame.

The inline tenon however is cut just like any other tenon and the angle is introduced only in the shoulder. A mortise gauge can be set once and used to do all the tenons and mortises thereby increasing accuracy considerably.

On the converse, the inline tenon requires an angled mortise. This scares a lot of people in both the hand tool and power tool camps. I submit that the joint isn’t hard to cut at all and when done by hand, no special sleds or jig (other than a scrap block) are needed. I grabbed a small black and planed in the angle of the splay then used it as a guide for my mortise chisel. One the initial pass is made with the chisel, chopping deeper is guided by the wall of the mortise already established. In the end, I found it no more difficult than chopping a square mortise.

Easier Layout, Same cutting difficulty, inline tenon is the way to go.

So with that difficulty removed and the layout being simpler, the inline tenon seems the obvious choice. Why worry about whether or not the tenon is stronger this way when cutting it isn’t any more difficult or time consuming than the angled tenon. What we know about wood says that the inline tenon should be stronger so to me why tempt fate?

compound angle tenon

How would you set up your tenon jig to cut this joint??

I think what really drives this debate is a difference between the woodworker reliant on power tools for accuracy and the woodworker who can cut a joint with hand tools. Angles mean nothing when you use a hand saw or chisel but trying to set up your machines to cut compound angles is another matter completely. I’m decidedly bias but I find this angle tenon debate a simple one to solve when viewed from the hand tool solution. Of course it helps that many of the chair makers I respect agree with me since I’m a novice when it comes to making chairs.

Your Turn

Do you find the angled tenon to be weaker than the inline tenon? Is the inline tenon actually harder to cut? Please share your experience and insight in the comment section below.

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Nik Brown

I had to cut all sorts of strange angled tenons when I made a 3 legged Chinese stool. If you do a through tenon and mortice it makes a difference due to the fact you want the exposed mortice to be centered on both sides.

This is the stool I’m talking about if you aren’t familiar with it: http://www.popularwoodworking.com/woodworking-blogs/editors-blog/chinese-stool-2-popular-woodworking-editors-0

It’s really a fun joinery challenge if you are ever searching for a fun little project. I made mine out of cedar and unless you are REALLY good at joinery I would say stick with a softish wood. Your joints would have to be dead on if you made it out of a really hard species… and it still might not go together.

    Shannon

    That’s a great project Nik and is on my list of things to try someday.

Jeremy

Good post. I agree with the points raised, that selection is primarily layout & tooling available (man vs machine) with a (personal) bias toward the in-line tenon.
One additional point to consider is assembly sequence. This choice may dictate assembly sequence and/or the (in)ability to sub assemble components when bringing multiple M&T joints together simultaneously.

    Shannon

    Great point Jeremy, a good way to break some joints is by forcing rails apart to fit into angled joints of sub-assembled pieces.

Kenny Comeaux

The bulk of my joinery cutting experience (at least until about 2 months ago) has been with power tools. That being said, I still have to agree whole heartedly with your view on this one. I wouldn’t hesitate to go hand tool and just cut the straight tenon with the angled mortise. Oddly enough (at least in my brain) it just seems like the simpler way to go. And when you add the fact that the straight tenon is obviously gonna be stronger, your way is a win/win.

Bob Rozaieski

Great post Shannon! Agree 100%. Inline is the way to go. I think coming from our power tool centric minds and backgrounds (I think even 99% of us who primarily/exclusively use hand tools started out using power)the angled tenon just seems easier because the angled mortise creates a lot of problems with jigs and ramps, etc. for making it with just about any kind of power tool. In some cases, like a router maybe, I can see making the angled mortise could be down right dangerous with power tools. The angled shoulder however becomes tricky with power. How do you get into the acute angle of the shoulder with any kind of power tool? I can’t think of any way at all (specially purpose ground table saw blade?). The angled tenon is easy on a machine though because the shoulder is all at 90 degrees to the stock face and edge.

With all that said, I think you are correct that the angled mortise is easier by hand. And while I have not had the opportunity to take apart many (read ANY) period chairs to see how they are assembled, all of the ones that I have seen disassembled had straight tenons and angled mortises, and all of the ones I’ve seen in museums where it could be determined how the mortise & tenon was cut (i.e. if it was a through tenon, visible from the back of the chair) were angled mortise and inline tenon. I’ve never seen an angled tenon on a period chair in the wild. So I think that lends a lot of support to your findings as well.

So whatcha makin’?

    Shannon

    I think when it is difficult to find something in the wild it isn’t an indication that it wasn’t done but more that it didn’t last. This may be one of those cases. All of the pieces I have examined have been in museums and they are of the Philadelphia school where the through tenon in the back leg reveals that it is inline. It would be interesting to look at some of the other regions and compare, especially chairs built outside the metropolitan areas.

    As far as what I’m building, it started out as merely an exercise as a way to wrap my head around these compound joints while I was filming a Hand Tool School lesson. Now I have several partially built chair frame made from random scrap pieces. I think it could be interesting to try and take them to at least a functional, if ugly, finish.

JeffDM

While more obscure than the pins vs. tails argument this one does seem to be heating up a bit! A few months ago Paul Sellers wrote an article which reached the exact opposite conclusion (itself a response to something in Fine Woodworking written by Jeff Miller who agrees with you). http://paulsellers.com/2012/05/more-flawed-concepts-from-fw/

He makes a strong assertion that all chairs everywhere for all time are made using angled tenons which is obviously not 100% correct given yours and Bob’s experiences. Maybe there is a UK vs US difference here.

    Shannon

    I probably should mention that Paul’s article was in my head when I began making all these joint. At the same time, there is quite a bit of pro inline tenon sentiment in the SAPFM forum from period makers whom I highly respect too. I am certain there are regional differences and I think differences will assert themselves in different styles of chair too. The later Arts and Crafts movement had different lines, wood species, and stock thickness than the more delicate Queen Anne so variations are bound to exist there too. I find furniture forensics to be fascinating and just how much you can ascertain about society and political happenings from the furniture style and how they were built.

Snakeye

Shannon,
I have to admit, from a hand-tool perspective, I think in-line method is the easiest. This is simply because it’s easier to use the build you have done so far to draw the non-square lines where the shoulders must be cut (against your actual joinery thus far), and again, it’s easier to saw straight down. But I like in-line because of the first reason – measure your cuts to your joinery off of the already formed parts that you’ve cut.
I’m an amateur though… and it’s developed as my personal preference.