Key Signature - Conventions

Conventions

In principle, any piece can be written with any key signature, using accidentals to correct the pattern of whole and half steps. The purpose of the key signature is to minimize the number of such accidentals required to notate the music. The sequence of sharps or flats in key signatures is generally rigid in modern music notation. For example, if a key signature has only one sharp, it must be an F sharp.

The effect of a key signature continues throughout a piece or movement, unless explicitly cancelled by another key signature. For example, if a five-sharp key signature is placed at the beginning of a piece, every A in the piece in any octave will be played as A sharp, unless preceded by an accidental (for instance, the A in the above scale — the next-to-last note — is played as an A♯ even though the A♯ in the key signature is written an octave lower).

In a score containing more than one instrument, all the instruments are usually written with the same key signature. Exceptions include:

• If an instrument is a transposing instrument
• If an instrument is a percussion instrument with indeterminate pitch
• Composers usually omit the key signature for timpani parts. Besides not using a key signature, timpani parts were early on also treated often as transposing instrument parts, the pitch of the high drum being written as C and, as timpani were almost always tuned a 4th apart, dominant on the low drum and tonic on the high drum, the pitch of the low drum being written as G, with the actual pitch indicated at the beginning of the part, e.g. timpani in D-A, if they were tuned A (low drum) and D (high drum)
• 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.