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韦 supplies the sound — wéi shifted slightly to wěi. Same phonetic appears in
围 (enclose),
伟 (great),
违 (disobey). The 'leather' meaning is unused here;
韦 is pure phonetic duty. The classical phrase
韦编三绝 (the leather bindings broke three times) hints at why this phonetic was reachable — both reeds and leather were everyday materials.