First off I know it's like zero deg. F outside, but, yes I'm working on a garden bench.
The plans are from an old Wood magazine and call for 3-1/4" and 4-1/4" x 1/2" fixed tenons where the seat rails meet the legs. The design has 6" tall rails so that's probably why the tall tenons.
I want to use my Domino XL for loose tenons. I will be making my own tenons. Looking at the original design I see I can fit three 12mm Dominos (that are 26mm wide) on top of each, but was just wondering since I'm making the tenons should I just overlap the Domino mortises and use one wide tenon - say 3" wide?