'read the side if any'), or dubanbian (读半边; 讀半邊; dú bàn biān; 'read the half'), is a rule of thumb people use to pronounce a Chinese character when they do not know its exact pronunciation.
Linguists rely heavily on this fact to reconstruct the sounds of ancient Chinese.
However, over time, the reading of a character may be no longer the one indicated by the phonetic part due to sound change and general vagueness.
For example, reading 詣 (pinyin: yì) as zhǐ because its "side" 旨 is pronounced as such.
For example, people read the character 町 ting in 西門町 (Ximending) as if it were 丁 ding.