勅修百丈清規 5.42

5.42

古僧衣律制只有僧祇支（此云覆膊衣。亦名掩腋衣）此長覆左膊及右腋。蓋𮍊三衣故。即天竺之儀也。竺道祖魏錄云。魏宮人見僧袒一肘不以為善。乃作偏袒。縫於僧祇支上相從。因名偏衫（今開脊接領者是。蓋魏遺制也）。

In ancient times, the vinaya rules for monastic clothing prescribed only the saṃkakṣikā (here translated as 'shoulder-covering robe', also called 'armpit-covering robe'). This covers the left shoulder and right armpit at length, covering underneath the three robes; it is indeed the Indian etiquette. The Records of Wei by Zhu Daozu says: 'When the ladies of the Wei court saw monks baring one arm, they did not regard it as good, so they made a single-shoulder covering and sewed it onto the saṃkakṣikā to accompany it; hence it was named pianshan' (the ones today with an open back and connected collar are these; it is indeed a legacy of the Wei dynasty).