What does a key signature indicate?

A key signature serves as a crucial element in music notation, indicating the key of a piece by specifying which notes are to be played sharp or flat throughout the composition. This system helps musicians understand the tonal center of the music and provides guidance on which notes will be altered from their natural state. For example, a key signature with one sharp indicates that F is raised a half step to F-sharp, and the piece is based around the tonal framework of G major or E minor.

The key signature does not communicate the tempo of a piece, which relates to how fast or slow the music should be played, nor does it convey emotional context, which is interpreted more through dynamics, expression, and style. Additionally, it does not indicate the number of measures in a piece, as this is determined by the overall structure and composition rather than the key itself. Thus, the primary function of a key signature is to define the scale and pitch alterations essential for interpreting the music accurately.