四分律行事鈔資持記 3.53

3.53

七中，初明揲角。疏云：相助為力故。又下，次明安紐揲肩。然須揲者，為障垢膩。疏云：數浣本衣，恐速壞故。

In the seventh [section], it first explains patching the corners. The commentary states: "Because they assist each other to provide strength." "Furthermore... below" next explains attaching the loops and patching the shoulder. The reason one must patch it is to block grime and sweat. The commentary states: "Lest frequent washing of the original robe cause it to wear out quickly."