Part 10 · Chapter 37

And behold, the upper Bina is called Samech, and the first Tevuna is called closed Mem, as above. And the matter is that the upper Bina is the measure of the ten upper Sefirot, which are Habad-Hagat, and since they are drawn from the secret of Chochma, which is Yud, it is found that the upper Bina is called Samech, since her place corresponds to her six upper Sefirot, each included of ten, totaling sixty (Samech). But Tevuna, which has only the measure of the four lower ones, each included of ten for the reason above, is therefore closed Mem. It is also called closed Mem because it sits fourth in the structure.