F major key is the home key of the English horn, the basset horn, the horn in F, the trumpet in F and the bass Wagner tuba. Thus, music in F major for these instruments is written in C major key. Most of these sound a perfect fifth lower than written, with the exception of the trumpet in F which sounds a fourth higher. (The basset horn also often sounds an octave and a fifth lower.)
The pitches or frequencies that make up the key of F are not as evenly spaced mathematically as other keys (in the western tuning system). This is means that the key of F is the hardest key to sing or play in because certain intervals are not perfectly spaced. This usually cannot be heard, but musicians often find it more difficult to stay on pitch when playing or singing in F.