What led to the historical predominance of the four-part harmony in Western Music?

Question

Introductory music theory heavily empasizes analysis and writing of music with four-part harmony, putatively the basis for music at the beginning of the common practice. Why did four-part voice leading, rather than, say, voice-leading in three, five or n parts, become the basis for much of baroque music and thereby become so heavily emphasized in music theory coursework?

Answer 1

The answer to the question of why four-part harmony is so popular stems from the development of counterpoint and its transition into what we now know as tonal harmony.

Counterpoint developed from plainchant, and the earliest type of counterpoint is known as organum, which began with only two voices. From two-part organum, the parts began to multiply. Soon, three and four-part harmonies were the norm. By the transition from Medieval to Renaissance, motets written in six voices were not uncommon.

Four-part harmony became the default with the development of the Chorale and the movement into tonal harmony. Chorale settings––which were a staple of Protestant services, and many of which were written by Martin Luther––were written almost exclusively in SATB form. With the heavy influence of Church Chorales, the choral arrangement SATB was codified.

Four-part harmony serves a practical purpose in tonal harmony as well: a minimum of four distinct pitches are necessary to make a seventh chord. So unless a composer wanted to use only triads or have certain chords implied, they needed at least four voices for something as simple as a V⁷ - I perfect authentic cadence.

Many larger choral works throughout the Common Period toyed with other orchestrations, but SATB remains the default for choral voicing to this day.