Rashi on Exodus 26:22/רש"י על שמות כ״ו:כ״ב

1ולירכתי AND FOR THE BACKPART — This is a term for “end”, as the Targum renders it, ולסיפי, “for the ends.” Because the entrance was on the east the east-side is called the front and the west-side the back. And this is why it is described here by a word denoting end, because the front is the head (the beginning) of a thing.

2תעשה ששה קרשים THOU SHALT MAKE SIX BOARDS — thus you have 9 cubits in the breadth.