Squaring the Housings
Three parts of the Tefillin must be perfectly square: the Bayit itself, the base, and the stitching. This is a law that originates at Mt. Sinai, without further explanation, as the Talmud states: "Square Tefillin - Halacha from Moshe at Sinai.... in their stitching and diagonally..."
We are punctilious about maximum precision in a square, even as much as tenths of a millimeter - less than the width of a hair on one's head.
The measuring is carried out by means of a digital caliper. The distances between the holes formed by the needle in stitching are measured in accordance with the ruling of Rabbi Mordechai Eliyahu Ztz”l, so that they will be equally spaced, in order to fulfill the precept, "This is my G-d and I will exalt him."