Where the shape comes from
A short horizontal with a dot resting on it — something laid over the top. It is never a word on its own; dictionaries simply call it tóu, "the head". It caps 六, 京, 高, 市, 交 and 亮, characters whose upper element was once a roof, a hat or a cover of some kind.
汉字 filed under 亠
No published character on HSK Guide is filed under this radical yet. 汉字库.