Now comes marking off the creases of the vertical piece. Working left to right in the illustration:

* The first mark is made at the distance L from one end
* The next mark is made the distance Th from the first mark
* From this mark, another L
* Now Th + 1 mm from the previous mark
* The remainder is left excessive

The piece now has a top and bottom; whichever end the T + 1 mm measurement is nearest is the top.