In music, a "KEY" refers to the group of pitches, or notes, that form the basis of a music composition. Each key is defined by its tonic, which is the first note of the scale, and can be either major or minor. The key determines the mood and character of the piece, influencing how it sounds and feels to the listener. Keys are often represented by a key signature, which appears at the beginning of a musical staff. This signature indicates which notes are sharp or flat throughout the piece. Understanding keys is essential for musicians, as it helps them interpret and perform music accurately, and is fundamental in music theory, including concepts like scales and chords.