同

胡同 is widely believed to be a borrowing from Mongolian during the Yuan dynasty, when Beijing was the imperial capital Dadu. The Mongolian word for a residential alley around a water-well (hottog) was transcribed into Chinese, and the tone shift of 同 preserved the foreign reading. The character was given its own traditional variant 衕 to mark the alley sense.

Use tòng strictly inside 胡同 and its compounds: 死胡同 (dead-end alley, also figurative impasse), 胡同儿 (the Beijing erhua form). Every other use of the character — 同学, 同时, 合同 — stays on the rising-tone tóng.