Flats are used in musical notation for several purposes. They can be used to create notes that are not normally found within the diatonic scale of the key in which a piece of music is written. This can be used to add chromaticism and create more complex melodies and harmonies. Flats can also be used to lower the pitch of notes in order to accommodate the vocal or instrumental range of a particular performer.
In jazz and other improvised music, flatted notes are sometimes used to create a "blue note" sound. A blue note is a note that is slightly lower than the corresponding major scale note. This can create a bittersweet, melancholic sound.
Flats can also be used in combination with other accidentals, such as sharps and naturals, to create more complex pitch alterations. For example, a double flat lowers a note by two semitones, while a flat-sharp raises a note by one semitone and then lowers it by one semitone.
Flats are an important tool for creating and understanding musical notation. They can be used to alter the pitch of notes, create new notes, and add chromaticism to a piece of music.