Charles Ellicott Commentary Exodus 26:17

"Two tenons shall there be in each board, joined one to another: thus shalt thou make for all the boards of the tabernacle." — Exodus 26:17 (ASV)

Two tenons. —By “tenons” here are meant projections, probably round, from the end of each plank, made to fit into holes prepared for them in the “sockets.” They were to be set in order one against another: i.e., placed regularly at certain intervals, so that each corresponded in position to its counterpart.