<Li> Composers may omit the key signature for horn and occasionally trumpet parts . This is perhaps reminiscent of the early days of brass instruments, when crooks would be added to them, in order to change the length of the tubing and allow playing in different keys . </Li> <Li> On occasion, in more modern works, instruments may be notated in different keys even when they are not transposing instruments, because the music is polytonal and the different parts are actually in different keys which sound together . </Li> <P> The convention for the notation of key signatures follows the circle of fifths . Starting from C major (or equivalently A minor) which has no sharps or flats, successively raising the key by a fifth adds a sharp, going clockwise round the circle of fifths . The new sharp is placed on the new key's leading note (seventh degree) for major keys or supertonic (second degree) for minor keys . Thus G major (E minor) has one sharp which is on the F; then D major (B minor) has two sharps on F and C and so on . </P> <P> Similarly successively lowering the key by a fifth adds a flat, going counterclockwise around the circle of fifths . The new flat is placed on the subdominant (fourth degree) for major keys or submediant (sixth degree) for minor keys . Thus F major (D minor) has one flat which is on the B; then B ♭ major (G minor) has two flats (on B and E) and so on . </P>

What key is f sharp and c sharp