brad, in that book the post are hard wood, Jack is using 1-1/2" mortises due to the fact that it is much harder to bore 2" holes in hardwood, if the post were softwood you would of seen a line 2" off the ref. face. He would not of snapped a line 1-1/2" off than calculated the extra 1/2" to the 2" mortise. On page 78 it clearly says "edge of mortise/tenon" with a arrow pointing to the line. Also notice that every thing is housed to 7-1/2" on a 8" timber, forming the inside face ready to receive the incoming member. That is only an example, on the actual frame he used a layout tool, based on the hard edge of the timber, no snap lines, see page 85 top left photo. Tim