Charles Ellicott Commentary Exodus 26:24

"And they shall be double beneath, and in like manner they shall be entire unto the top thereof unto one ring: thus shall it be for them both; they shall be for the two corners." — Exodus 26:24 (ASV)

They shall be coupled together beneath. —The corner boards were to be coupled to the others in two places, below and above, in each place by means of one ring. Rings, through which the ends of the bars mentioned in Exodus 26:26-29 passed, are understood to be what is meant.