During the Han Dynasty, the emperor's commode was called 虎子 (hǔzi). However, the grandfather of Emperor Gaozu of Tang was called 李虎 (Lǐ Hǔ). Because it was forbidden for common items to contain the given names of members of the imperial family (see Naming taboo), 虎子 was changed to 馬子／马子 (mǎzi), which morphed later into 馬桶.
