~中 (じゅう vs ちゅう)

The kanji 中 reads じゅう ('throughout, all over') after spatial or temporal extents (一日中 'all day', 町中 'throughout the town') and ちゅう ('during, in the middle of') after activities or states (仕事中 'at work', 食事中 'while eating'). Selection depends on the preceding noun's nature.