Charles Ellicott Commentary Exodus 26:8

"The length of each curtain shall be thirty cubits, and the breadth of each curtain four cubits: the eleven curtains shall have one measure." — Exodus 26:8 (ASV)

Thirty cubits. —The additional cubit on either side would hang down and form a “valance” along the sides of the tent. (See Exodus 26:13.)