In the May 2015 issue of WOOD Magazine, on page 50 there is a plan for a Woven-Seat bench. In the diagram (which I can't seem to find online, it may be copyright content) the short and long rails which form the basis of the bench seat are cut such that the tenons are cut with a 45 degree miter. Why would someone do this? What problem might that solve or advantage would it convey?
Why would someone do this? What problem might that solve or advantage would it convey?
If you have two rails joining a leg at right angles to each other, you sometimes need to miter the tenons to prevent them from interfering with each other.
I haven't seen the bench you're talking about, but it sounds like a typical case where you have rails connected to legs with mortise and tenon joints. If the tenons on the rails reach to the center of the leg, they will of course bang into each other when they're inserted into the mortises. To prevent that, you miter the ends of the tenons.
An alternative would be to shorten one or both of the tenons so that they don't reach so far into the leg, but shorter tenons mean joints that have less resistance to racking forces. Mitering the tenon shortens just the inside cheeks while allowing the outside cheeks to stay long, providing a little more mechanical strength and glue surface.
Update: There are some drawings of table-rail joints with the tenon ends mitered in this article at the heading Three-dimensional Frames.