Write two melodic lines to be sung or played at the same time, and classical counterpoint imposes a specific, oddly narrow-sounding restriction: the two lines are generally forbidden from moving in "parallel fifths" or "parallel octaves" — maintaining the exact same harmonic distance from each other, in the same direction, from one note to the next. It sounds like an arbitrary rule from a music theory textbook. It's actually rooted in something audible.
Why two lines moving in lockstep stop sounding like two lines
A perfect fifth and a perfect octave are the two intervals whose component notes blend together most completely, sharing so much of their overtone structure that the ear has genuine trouble perceiving them as fully separate pitches rather than one thickened tone. Move two independent melodic voices in parallel fifths or octaves, and for the length of that motion, the two lines stop sounding like two independent voices in dialogue and start sounding like one voice doubled — exactly the effect counterpoint, built around maintaining audible independence between voices, is designed to avoid.
A rule with room for exception, not an absolute ban
This isn't an unbreakable law of music — composers use parallel fifths and octaves deliberately for specific effects throughout music history, and some styles, certain folk and medieval traditions, some twentieth-century composition, embrace exactly the thick, blended sound classical counterpoint avoids. Within the specific tradition of species counterpoint, developed as a structured teaching method and most associated with the eighteenth-century theorist Johann Joseph Fux, avoiding parallel fifths and octaves is treated as a foundational discipline — a constraint meant to force a student's ear to notice, and preserve, the independence of each voice before they're free to break the rule on purpose.
What we're still unsure about
The acoustic explanation for why parallel fifths and octaves specifically sound like voice-merging, rather than some other interval, is well grounded in overtone physics, but music theorists still debate how much of the rule's centuries-long persistence in Western training reflects a genuinely universal perceptual effect versus a culturally reinforced convention passed down through a specific pedagogical tradition. Some other musical cultures use parallel harmony extensively without treating it as a problem to avoid, which complicates any claim that avoiding it is simply what sounds objectively "correct" to human hearing in general.
This sits inside Counterpoint & Voice Leading, one of eight topics in Music Theory, one of four domains in Music, one of seventeen subjects the app can quiz you on.