The word perfect

[joefb60]

Why are unisons, 4ths, 5ths and 8ths called perfect?

[franz_]

Why are unisons, 4ths, 5ths and 8ths called perfect?

Who says that?

[prometheus]

Because they are.

Thirds aren't perfect. A major third is the most constant. But a minor third is no dissonant. A perfect fifth is very constant and a step up or down and it is very dissonant. That is the difference.

[pianistimo]

i think it has to do with the pattern of whole and half steps on the keyboard.  for diatonic scale of C:  F (the fourth) would have no 'minor fourth' to speak of because there is no black note there.  u do have an augmented fourth (F#).  since that is already taken - the fifth can be perfect as well because there is no need to name something twice.  minor sixth takes care of the A-flat.  octaves are unison.  that is as perfect as an interval can get.

if u take each of the diatonic notes of the scale of C and place thirds atop them until u have a three note chord - u have certain CHORDS of the scale.  this is why C major chord (C E G) =I
d, f, a = ii  and so forth.  because these chords are predetermined by the diatonic scale - then u also modify them accordingly.  it still has to do with the basic scale set-up, imo

[pianistimo]

the overtone series probably has something to do with all this, too.

[timothy42b]

Not really correct, I'm afraid.

The word perfect predates the use of major and minor scales, and depending on your definition does not apply to modern temperaments. 

So the term is an unfortunate holdover from an earlier age that adds no additional information and is confusing.  I don't use it.  I just say fifth.  You know it isn't augmented or diminished or I would have added that.  You also know it isn't "perfect" under equal temperament. 

Under Pythagorean temperament the fifths and fourths really were perfect in the sense of A) being true small integer rations and B) sounding beatless.  But all the other intervals are awful and Pythagorean sounds really bad.  Under equal temperament the errors are spread evenly, and no interval is perfect.  But of course on the piano, because of inharmonicity, pretty much no interval is beatless regardless of how you tune it. 

[pianistimo]

agreed about the perfect idea of a perfect interval. 

in the new harvard dictionary it explains that in 'western nomenclature' each interval has a number of semi-tones that make it a certain interval.  the number of semitones between the two pitches is indicated by a qualifying adjective (perfect, major, minor, diminished, or augmented).  'the fourth, fifth, and octave are called perfect...aperfect interval if reduced by a semitone becomes diminished...a major interval if reduced by a semitone becomes minor...and a minor interval if f reduced becomes diminished.  both perfect and major intervals become augmented if increased by a semitone.'