Numbers 4:6

Put in the bars. Hebrew, "place the bars," upon the shoulders of the Levites; for they were never taken out of the sides of the ark, Exodus xxv. 15. Other bars, like hand-barrows, were used to carry the ark and the other different vessels, after they were folded up. Two Levites bore them on their shoulders. (Calmet) --- Perhaps the bars of the ark, after it was enveloped with the three curtains, might project, so that the bearers might take hold of them. (Menochius)