The tabernacle has walls of gold-plated wood (Exodus 26:15-25). Those walls are held together by gold-plated bars (26:26-30).
There are five bars on each of the long sides of the tabernacle (vv. 26-27), and five bars across the back wall (Exodus 26:27). The bars are held in place by gold rings attached to the boards; the bars are slid through, as the poles are slid through the rings on the tabernacle furniture (see here).
The central bar is "in the midst of the boards shall pass through from end to end" (Exodus 26:28). Some have thought this means that holes were bored width-wise into the boards, so that the middle bar could pass through the "tunnel." That doesn't seem plausible, given the dimensions of the boards. Instead, the text simply means that the central bar is continuous, from one end of the wall to the other, held up by the rings on each board.
That central bar is singled out for a reason. It "anchors" the whole assembly of furniture in the tabernacle, and conceptually matches the poles used for carrying the furniture. As I mentioned in an earlier discussion, the rings are placed differently on the different pieces of furniture: The rings of the ark were at the "foot," the rings of the table were just under the tabletop. When hoisted on the shoulders of Levites, the ark would be elevated above the bearers and the table would be mainly below the shoulders of the bearers. The rings of the altar were near the middle. Together, the three formed a stepladder from table to altar to ark.
Now, with the bar through the middle of the walls, this stepladder remains part of the tabernacle even when it is at rest. The rings of the furniture aren't the same rings as the rings of the walls, but they match architecturally. It's as if the central bar holds up the table, the incense altar somewhat higher, and the ark at the peak. Though the furniture of the tabernacle rests on the ground, there is a symbolic ascent toward the throne of Yahweh.
One last, speculative note. The word for "bar" is beriyach, often used to describe locks on doors or bars on city gates (Deuteronomy 3:5; Job 38:10). That suggests the tabernacle is a "fortress" as well as a sanctuary. And the word itself echoes with other Hebrew words - berith, "covenant" and ruach, "spirit." Perhaps the bars of the tabernacle highlight the tabernacle's place as a covenant symbol, and as a temple of the Spirit.
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