Exodus 26:18-21

And thou shalt make the boards for the tabernacle, twenty boards on the south side southward. And thou shalt make forty sockets of silver under the twenty boards; two sockets under one board for his two tenons, and two sockets under another board for his two tenons. And for the second side of the tabernacle on the north side there shall be twenty boards: and their forty sockets of silver; two sockets under one board, and two sockets under another board.