C. F. Keil and Franz Delitzsch on Exodus 26:28-29

Exo 26:28-29

“And the middle bar in the midst of the boards (i.e., at an equal distance from both top and bottom) shall be fastening (מבריח) from one end to the other.” As it thus expressly stated with reference to the middle bar, that it was to fasten, i.e., to reach along the walls from one end to the other, we necessarily conclude, with Rashi and others, that the other four bars on every side were not to reach the whole length of the walls, and may therefore suppose that they were only half as long as the middle one, so that there were only three rows of bars on each wall, the upper and lower being composed of two bars each.