The ancients used 'female and male' to represent yin and yang.
Tips
history
Classical animal-husbandry terminology: 牝 = female, 牡 = male. Famous in 列子's story 九方皋相马: when judging horses, sage Jiufang Gao 'forgot the 牝牡' (sex) and 骊黄 (color), seeing only essence, the proverb for grasping substance over surface.
memory
牝 has 匕 (a 'feminine' curve) on the right; 牡 has 土 (solid 'masculine' earth) on the right. Both share the 牛 (cow) radical, these are originally cattle terms.
