שְׁתֵּ֣י יָד֗וֹת לַקֶּ֙רֶשׁ֙ הָאֶחָ֔ד מְשֻׁלָּבֹ֔ת אִשָּׁ֖ה אֶל־אֲחֹתָ֑הּ כֵּ֣ן תַּעֲשֶׂ֔ה לְכֹ֖ל קַרְשֵׁ֥י הַמִּשְׁכָּֽן׃
Two tenons shall there be in each board, joined one to another; thus shalt thou make for all the boards of the tabernacle.
Read kabbalistic (mystical) interpretation of Exodus 26:17: and classical mystical sources.