Where the shape comes from
One of the eight fundamental brush strokes, a blade sweeping down from upper right to lower left. Too simple to picture anything on its own, it gathers characters built around this single motion — 九, 乃, 乎 — and appears doubled and crossed in 乂. Many of the characters filed under it are ancient grammatical particles whose original pictures were lost long before the Kangxi dictionary sorted them by shape alone.
汉字 filed under 丿
No published character on HSK Guide is filed under this radical yet. 汉字库.