Hi

I imagine this is a very stupid question but please bear in mind that I am a complete amateur.

I am preparing to try my first mortise & tenon joint with a small shelving unit and I was just wondering why the tenon can't be left the same thickness as the rest of the shelf? I see that the general rule seems to be to make it a 1/3 of the thickness but I don't understand why and I would be happy to leave the tenons at their current thickness if it isn't going to weaken the shelf.

Thank you in advance for your time

Best wishes

Ian