John Gill Commentary Ezekiel 41:9

"The thickness of the wall, which was for the side-chambers, on the outside, was five cubits: and that which was left was the place of the side-chambers that belonged to the house." — Ezekiel 41:9 (ASV)

The thickness of the wall, which was for the side chambers without, was five cubits This is the outward wall of the chambers, north and south, which was five cubits thick, made of stone; (See Gill on Ezekiel 41:5), and that which was left was the place of the side chambers that were within; this was a void space, not built upon, which was before the chambers that stood within it; and was a space to walk in for those that dwelt in the chambers, or to go in from chamber to chamber; which also was five cubits in breadth, as appears from the next verse. This may denote the communion of churches, and the members of them one with another.