Charles Ellicott Commentary Exodus 26:22

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

For the sides of the tabernacle westward. —Rather, for the back of the tabernacle (Septuagint, τῶν ὀπίσω). (See Note on Exodus 26:18.) The west is always regarded as “behind” by people in the East.

Six boards. —Six boards, presumably of the same width with the others (Exodus 26:16), would extend a length of nine cubits only, or thirteen and a half feet. The tenth cubit seems to have been made up by the corner boards, or posts, which are counted with the “six” boards as forming the back of the tabernacle in Exodus 26:25.