The coro has fixed melody and lyrics. These can change during a piece, though, and are often made up on-the-fly, but within one section, they are repeated unchanged. Coros are usually homophonic, but not necessarily in unison. Most coros feature two or three parts, rarely more, moving in parallel lines. The parts may or may not be doubled, according to the artist's preference and possibilities. Some genres like rumba prefer a more massive coro sound and double parts whenever possible, while other genres like salsa dura prefer the clean sound of a small coro.