Exodus 38:5
They cast bronze rings for the four corners of the bronze grating to hold the poles. The four bronze rings at the corners of the grating give the grating its own portable mechanism within the altar. The grating that sits inside the altar can be removed for cleaning or transport via its own poles. The layered portability of the tabernacle system — the altar with its grating, the grating with its own rings — is the engineering of a worship system designed for a mobile community. Every element can move when God moves.