What does a key signature indicate in a piece of music?

A key signature is a critical element in music notation that indicates the key of a piece by designating which notes are to be played as sharp or flat throughout the piece. This provides musicians with essential information about the tonal center of the music, guiding them in understanding which scale or mode governs the composition. For instance, a key signature with one sharp indicates that the piece is in either G major or E minor, while a flat key signature may denote F major or D minor.

Understanding the key signature allows performers to interpret the music accurately, ensuring they maintain the correct pitch relationships as intended by the composer. By knowing which notes to alter, musicians can better grasp the harmonic structure and emotional quality of the piece, shaping their performance accordingly.

The other choices either pertain to different aspects of music (such as tempo and dynamics) or refer to genre characteristics, which do not relate to the function of the key signature.