While comics are often the work of a single creator, the labour of making them is frequently divided between a number of specialists. There may be separate writers and artists, and artists may specialize in parts of the artwork such as characters or backgrounds, as is common in Japan. Particularly in American superhero comic books, the art may be divided between a penciller, who lays out the artwork in pencil; an inker, who finishes the artwork in ink; a colourist; and a letterer, who adds the captions and speech balloons.