Exodus 38:7
Then he inserted the poles into the rings on the sides of the altar for carrying it. He made the altar with boards so that it was hollow.
Berean Standard Bible
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And he put the staves into the rings on the sides of the altar, to bear it withal; he made the altar hollow with boards.
He put the poles into the rings on the sides of the altar, with which to carry it. He made it hollow with planks.