千

Ten radical (Kangxi 24) — the indexing radical. Mnemonic: ten 十 with a 'cap' marker = a hundred tens = one thousand. The radical also helps differentiate 千 from siblings 干 (dry) and 午 (noon), which differ only in the topmost stroke.

Top-leaning marker stroke — historically a mark added to 人 (person) to denote a count of one thousand 'people'. The single sweep is purely a numerical marker, distinguishing 千 from 干 (which has a horizontal top instead). It carries no real-world picture, only the abstract 'this is the thousand-glyph'.