 They are usually in the keys of F or B, although many F horns have longer slides to tune them to E, and almost all B horns have a valve to put them in the key of A.The problem with single horns is the inevitable choice between accuracy or tone – while the F horn has the "typical" horn sound, above third-space C accuracy is a concern for the majority of players because, by its nature, one plays high in the horn's harmonic series where the overtones are closer together.This led to the development of the B horn, which, although easier to play accurately, has a less desirable sound in the mid and especially the low register where it is not able to play all of the notes.