The rule in Ellis’s book is two-fold. First, make the tenon one-half the width of the rail you’re cutting it on (a 2″-wide rail would get a 1″-wide tenon). Second: If that tenon’s width would be greater than six times its thickness, then you should split it into two (or more tenons). Example: You want to cut a 1/4″-thick tenon on a 6″-wide rail. Ellis’s rule says that your tenon should be 3″ wide. But a 3″-wide tenon is greater than 1-1/2″, which is six times the tenon thickness. So you have to break that tenon into two 1-1/2″-wide tenons.