John Gill Commentary Exodus 26:22

"And for the hinder part of the tabernacle westward thou shalt make six boards." — Exodus 26:22 (ASV)

And for the sides of the tabernacle
Or the ends of it, the east and west, and the account begins with the west:

westward you shall make six boards ;
so that the breadth of the tabernacle was but nine cubits, or four yards and a half, according to a common cubit; but two boards more placed at the two corners of the sides, next observed, added to the breadth of it.