I have a question I can’t seem to find the answer to: why do musicians tend to favour four-measure-long phrases?
I can’t speak for non-Western music, but four-bar phrases seem to exist in almost all Western popular music and the majority of music from the common practice period. I’ve been thinking about irregular phrase lengths in popular music, but I realized I don’t know why the tendency towards four bar phrases exists in the first place. Aside from just following the musicians before us and writing what we’re used to hearing, is there a psychological or musical reasoning for this?-L.
This is a really good question, and is one that theorists have been considering for centuries. Four bar phrases (or depending on tempo, 2 or 8 bar phrases) are very common in Western tonal music, but there’s no real reason why these duple groupings of measures seem to be privileged. Various theorists and music philosophers throughout history tried coming up with all kinds of rationalizations, but none of them really made sense; some argued that it was based on the duple nature of the human heartbeat, or the bilateral symmetry of the human body, and that was the reason for the “natural” use of duple groupings. Those were all predicated on the assumption that there’s an innate preference in human beings for duple organization and for symmetry; there’s evidence in favor of our preference for symmetry in general, but evidence of a duple preference is more mixed and gets complicated by the fact that (as you mention) a lot of music we hear has a duple organization so it’s hard to know if music is written that way because we prefer it that way, or if we prefer it that way because so much of it is written that way.
Using four measure phrases creates a sense of regularity or structure at a different level than that of the meter; if we consistently hear four measure phrases it creates a larger pattern where we start to expect some kind of cadence or closure. In his book Phrase Rhythm in Tonal Music, William Rothstein refers to this as phrase rhythm, and describes many ways in which composers can alter phrase rhythm for musical purposes. If you’re used to hearing four measure phrases, and suddenly there’s one that’s shorter or longer, it draws your attention to it and disrupts the regularity at the phrase level.
There are definitely musical traditions where these aren’t the norm. In fact, Western composers would often write irregular phrase lengths when trying to evoke some kind of a non-Western “other” in their music; the fourth movement of Brahms’s Piano Quartet in G minor. Op. 25 is called the Rondo alla Zingarese, or what would have been known then as “in Gypsy style.” (That term is problematic, and today we refer to that culture as “Romani” instead.) To evoke the dramatic character of traditional Romani violin music, Brahms included phrases that were only three measures in length, creating a perceptual quickening of the phrase rhythm, or how often we hear a complete phrase completed.
Another possible reason is that depending on tempo, a group of four measures probably falls within the window of time that we can remember, and perceive the phrase as connected as a musical idea; we can remember the beginning, middle, and end of the phrase and melody and feel as though we’ve heard a complete thought. This might be why at slower tempos we sometimes see two measure phrases instead of four measure phrases — if the tempo is slow, a four measure phrase will take longer, which may exceed our ability to remember and interpret the musical thought as one unit.
I hope that helps!