What was the significance of the middle bolt?
Rashi and Rashbam #1: The middle bolt, which ran inside the wall from one end of each side to the other, measured thirty Amos inside the north and south walls, and twelve Amos inside the west wall. 1
Rashbam #2 (citing Shabbos, 98b): The middle bolt - miraculously - went round all three sides 2 of the Mishkan. 3
Targum Yonasan: The middle bolt, which went round three sides of the Mishkan by means of a miracle, 4 angels brought from the trees that Avraham Avinu planted in Be'er-Sheva, and deposited in the Yam-Suf when Yisrael crossed it. Yisrael took it and used it for the middle bolt of the Mishkan. 5
Rashbam: Thereby joining the west wall with the north and south walls.
Rashbam: The north, the west and the south. This constituted a miracle, as the Gemara states in Shabbos, 98b.
Riva (in Pasuk 27): Rashi's explanation follows the Beraisa of Meleches ha'Mishkan, that there was a separate middle bolt on each wall - as opposed to the Gemara. See Torah Tem imah, note 24.
It turned the corner - like a snake, whilst being inserted and straightened itself - like a stick, whilst being withdrawn (Targum Yonasan).
In keeping with the Pasuk in Tehilim (89:3) 'The world is built on Chesed' - the Midah that prompted Avraham to plant the trees for the benefit of others.
