Which intervals are considered 'perfect' intervals?

Answer 1

The Perfect Intervals are the perfect unison which is playing the same note in the same octave ex: C4 to C4. the Perfect 4th which is 5 half steps ex: C to F , the Perfect 5th which is 7 half steps ex: C to G, and lastly the Perfect octave (Perfect 8th) or 12 half steps ex: C4 to C5.

Answer 2

Again, Julian's answer is excellent. I would also add that only perfect intervals remain perfect when they are inverted - in other words a perfect interval's compliment is also perfect. For example, C to F is a perfect 4th. When that interval is inverted and becomes F to C it is now a perfect 5th. When inverted, minor intervals become major (and vice versa), and diminished intervals become augmented (and vice versa). Perfect intervals have the moniker "perfect" not only for their consonant sound (in most cases), but also because they do not change their quality when inverted. A major third, for example, will have a consonant sound (in most cases), but the major 3rd's quality changes to minor when it is inverted. It is then a minor 6th.

Boom, music theory!

Answer 3

Perfect intervals are classified by their consonant sound. The intervals labeled as perfect intervals are the perfect prime/unison, perfect 4th, perfect 5th, and the perfect octave. However, the perfect 4th may not be consonant in some music.