I’m a big fan of hand-chopped mortises. There’s something about the seemingly-contradictory requirements of power and finesse that appeals to me. The problem with them is that, unlike drill presses, there is no depth stop to keep you from happily chopping your way right out the other side of the board.
The most common way suggested to prevent this is to wrap a piece of tape at the appropriate point on the chisel blade, and stop when you reach it. The problem with this method is that you rock the chisel to lever chunks out of the mortise as you work your way deeper. Invariably, my tape ends up looking like the example above.
I have found a better solution in the Sharpie permanent marker, which isn’t as permanent as you might think. A simple line on the chisel blade, and you’re set to go with a mark that won’t wiggle and bunch out of place. When you’re through, a wipe with denatured alcohol leaves you with a pristine chisel blade again.
So much for permanent.