Exodus 26:17

Two joints shalt thou make in one post, answering the one to the other: so shalt thou do to all the posts of the tabernacle.

Two tenons shall there be in one board, set in order one against another: thus shalt thou make for all the boards of the tabernacle.

There shall be two tenons in each board, joined to one another: thus you shall make for all the boards of the tabernacle.