The batim of tefilin must be square in 3 places: the ketzitza (the “box” that houses the parshi’os); the titura (the base under the ketzitza – not including the ma’avarta where the retzu’os pass through); and the stitching. If any of these are not square, the tefilin are pasul. The “squareness” does not necessarily need to be measured with calipers or a micrometer – if it looks square to the eye that is enough, at least bidi’eved. Even so, batim-makers usually use calipers to get the measurements as precise as possible. There is no requirement for the height of the ketzitza – it need not be a cube, so long as the length and width are equal.
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STAM 101 introduction & index
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more about square