Help! How do I make two mortise/tenons join at 90 degrees?

I'm building my first project, a workbench, and trying to do it all with joinery. The legs are 4x4s (actually 3 3/8") and so are the stretchers that will connect them.I've been reading basic rules for tenons (width ~1/3 the tenon, length 5x the width, etc). But if I'm joining two tenons into the same leg at 90 degrees, what should I consider?I've read about giving them a 45 degree chamfer, but not super confident I'll get that measured properly to fit right. Any suggestions appreciated! via /r/woodworking http://ift.tt/2taGoMB