What is the purpose of a key signature in music?

The purpose of a key signature in music is to provide a set of sharps or flats placed at the beginning of the staff, indicating the key of the piece. This helps performers understand which notes will be consistently raised or lowered throughout the composition, allowing them to interpret the music correctly. By defining the tonality, a key signature establishes a framework for both melody and harmony and guides musicians in their playing or singing.

For example, if the key signature has two sharps, it indicates that the piece is in the key of D major or B minor, and performers will know to consistently play F# and C# unless otherwise indicated by accidentals. This foundational element supports the overall structure of the music and is crucial for accurate performance.