How do choirs divide singers into soprano, alto, tenor, and bass parts?

In choral music, the division of singers into soprano, alto, tenor, and bass (SATB) parts is primarily determined by their natural vocal range and voice type. These classifications are rooted in biological factors, specifically sex and the physical dimensions of the larynx, which influence the register in which a singer can most comfortably and effectively produce sound.1

• Soprano: The highest vocal register, typically assigned to female voices (or boy trebles), covering the range from approximately middle C upward.2
• Alto: Derived from the contratenor altus of early polyphonic music, this part occupies the range below the soprano.3
• Tenor: Historically the part that "held" the melody, it is now the highest standard male vocal range, situated between the alto and bass.3
• Bass: The lowest vocal range, typically assigned to male voices with larger laryngeal dimensions.1

In four-part writing, these voices are arranged to balance the harmonic structure, with the soprano carrying the highest line and the bass providing the foundation.23