In making a tenon there are three variables that the woodworker has at his disposal to modify; they are length, width and thickness. Between tenon width and tenon length I’ve often wondered which of the two variables played a larger role. It seems logical that increasing either variable will lead to a stronger mortise and tenon joint – this isn’t the interesting part, but rather which one, of the two (length and width), plays a larger role? Fine Woodworking (#259–Jan/Feb 2017) recently published an article that provides a decent amount of data to answer the question; however, more data points would have been better. The article is an excellent read and I’d encourage any woodworker to read it. My biggest problem with the article is that the author made an error in his conclusion regarding the relative role between tenon length and width in how it relates to strength. The author concluded that the tenon width had the largest effect, but this isn’t accurate when you plot both sets of data and compare the slopes of both lines. Smaller changes in tenon length, when compared to tenon width, have a larger effect on joint strength. So, in the end, tenon length rules the day, but tenon width still plays a large role. I have the plots below in one graph (blue is tenon width, and orange is tenon length), but I blurred out the axis values because it’s not my data to publish – you will have to read the article for all the details.
It’s still a very cool article and goes into other aspects of mortise and tenon strength that are certainly worth reading.