Exodus 26:24

And they shall be coupled together beneath
At the bottom of the boards or planks:

and they shall be coupled together above the head of it unto one
ring
at the top of the boards there was a ring, to which they were coupled and fastened, and so at the bottom of them, though not expressed: some understand this of all the planks, both at the two sides and at the west end; but it seems rather to respect only the corner planks, since it follows:

thus shall it be for them both, they shall be for the two corners;
which were coupled and joined alike at each corner as if they were twins, as the word used signifies; or the meaning is, that these were double boards, and so like twins, which were so closely put together that they seemed but one board; which was done that the corners might be thicker and stronger, and so for the greater firmness of the building.