I decided to start the mortise of my very first mortise and tenon joint today. I don’t have a mortise chisel, so I used the old “drill holes and chop out the waste” method with my Millers Falls #5.

It’s 13/16″ deep and a little more than 1/4″ wide.
Surprisingly, this didn’t take much time at all. I think I spent a much more significant amount of time fiddling around with sharpening my chisels than doing the actual cutting, but that was only because I was being a bonehead with my 1/4″ chisel.
Even more surprisingly, it looks halfway decent. The sides seem reasonably flat and perpendicular. I guess you only get to see how badly you messed up when you cut the tenon, which is what I have to do next.