Comment by stephencanon

The usage of “perfect fifth” to indicate only the just fifth (3:2 ratio) is slightly idiosyncratic (but not unheard of); more commonly in modern usage, “perfect fifth” refers to both the just (3:2) and equal-tempered (2^(7/12)) fifths, or to any other tempered fifth, for that matter (for reasons that aren’t entirely clear—or at least differ depending on who you ask—the unison, fourth, fifth, and octave are collectively called the “perfect” intervals in common parlance, regardless of temperament).