Perhaps this question should be a separate thread, but I'll chime in here with a "why do it this way?" question I encountered after sewing the MacWerner Watch per The Book's instructions.

When joining two lengths of tartan, you're told to create a flat-fell seam that lies midway between the inner and outer crease of a pleat. The seam extends from hem upward to the point where the excess material is later cut away. During final pressing, I found that this seam created a slight bulge in the pleat from fell to cut-away.

On a previous hand-sewn wool kilt (done to other instructions) I had placed the join right at the inner crease of a pleat. This leaves raw edges inside the kilt, but I did a hand-overcast stitch which has kept them neat. Granted, when you start tapering that pleat the join will no longer be right at the crease -- but it doesn't create the bulge that the flat-fell does.

Pros and cons for each method, but am I missing some other rationale for using Barb's method vs. "mine"?