And this is the mystery of the name of AV (72), which is contained in the four names of AV, SaG, MaH and BaN. This is already quite clear from the teachings of Rabbi Shimon bar Yochai and the holy ARI. For each of the four names contains 72 in a different way.
In the case of the expansion of AV itself, 72 is the simple sum of the numerical values of its constituent letters. In the case of SaG (=63), we reach 72 by also reckoning an additional ten units for the ten individual letters of the expansion aside from their numerical value. (63 + 10 = 73; the extra unit is the kolel signifying the overall unity.) In the case of MaH (=45), we reach 72 when we reckon the numerical value of the simple name (=26) without adding units for the letters. Thus 45 + 26 = 71 (and one extra unit for the kolel brings the total to 72). In the case of BaN (=52), we reach the total of 72 when we reckon the fourfold form (ריבוע, ribu’a) of the Name, i.e. Yud (10) + Yud-Heh (15) + Yud-Heh-Vav (21) + Yud-Heh-Vav-Heh (26) = 72.