The dū reading is the original sense - in classical texts a 都 was a city that held a ruler's ancestral shrine, ranking it above an ordinary 邑. The modern senses 'capital' and 'metropolis' both descend from this royal-city meaning.
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Spotting the dū reading: it is always a noun and almost always sits in a city or place name - 首都, 都市, 京都, 成都. If the word is a city or 'capital', switch to dū; everywhere else default to dōu.
Right indexing radical - right-side 阝 is a compressed 邑 'walled city.' Distinct from left-side 阝, which represents 阜 'mound.' Puts 都 in the city-and-region family with 邦, 郊, 郡.
Left phonetic 者 - supplies the sound; zhě drifted to dū / dōu, opaque in modern Mandarin but consistent with related city characters. Originally a graph of simmering food or sugarcane, here purely a sound anchor. Kept as a single component since 者 is itself a recognized standalone unit.
Sichuan's capital and a benchmark dū-reading place name. Famous for hotpot, pandas, and tea houses; the 2017 song 《成都》 by Zhao Lei made the name a touchstone of Chinese indie pop.
