The secondary crease can now be marked on the first flap.

The flaps are marked successively; each is marked, creased, and folded over into closed position before the next one is marked. The reason for this is that the total thickness of the book and enclosure gains one board thickness with the closing of each flap, so the T measurement is accurate only for the first flap.

If you took the T measurement in the first step, simply transfer the measurement to the flap. If you did not measure, you can arrive at the T measurement as shown. Be careful not to mark on the book itself. (In fact, see the next slide before proceeding.)

Flaps are folded in the following order, relative to the orientation of the book:

1. bottom
2. top
3. right
4. left