Perfect fourth
From Xenharmonic Reference
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The perfect fourth is the octave complement of the perfect fifth. It generally has a frequency ratio of 4/3, and is 498.0 cents in size when justly tuned.
While it has a relatively simple ratio (and can thus be used as a consonance especially in Pythagorean theory or as a harmonic interval), in Western classical theory (Baroque and after), the perfect fourth above the root is often treated as a dissonance to resolve down to the major third.
