How did quarter-tones and microtonal music expand the pitches available to composers?

In the context of musical composition, quarter-tones and microtonal music expand the available pitch palette by moving beyond the constraints of the standard equal temperament system.12

While the Western chromatic scale divides the octave into 12 equal semitones (100 cents each), microtonal music utilizes intervals smaller than these half-steps. Composers achieve this expansion by incorporating:12

• Fractional Intervals: Theorists and composers have explored divisions such as quarter-tones (50 cents), sixth-tones (33.3 cents), and even smaller increments like 12th or 16th tones.1
• New Tonal Resources: By utilizing these smaller intervals, composers can access a vast array of pitches that do not exist on a standard piano keyboard.12
• Specialized Instrumentation: Because traditional instruments are often built for the 12-tone system, some composers, such as Alois Hába, designed custom instruments to perform microtonal scales consistently.12
• Extended Techniques: Performers on string and wind instruments can also produce these pitches by adjusting their finger placement or embouchure to play "between" the standard chromatic notes.32

These techniques allow composers to seek new means of expression, effectively breaking the limitations of the traditional 12-tone scale.3