Which book are you reading?

Unless I'm mistaken the chalk line is just a reference line. It doesn't mean that this is the side of the tenon unless the layout is 1 1/2" off the true outside corner and then the mortise is 1 1/2" wide.
If the layout was 2" x 2" then you'd layout the mortise 1/2" off this 1 1/2" chalk line every where long this timber.
Does that make sense?


Whatever you do, have fun doing it!