Perfect fifth interval

Perfect fifth in musical notationThe perfect fifth interval consists of two notes with a distance of seven semi-steps. For example, C to G note will result in this musical interval.

The reason behind the name “perfect” goes back to the Medieval. The unison, fourth, fifth and octave were considered most consonant and were given the name perfect. The perfect fifth interval is abbreviated P5 (an alternate spelling is diminished sixth).
Keyboard perfect fifth interval 
A perfect fifth C to G on the keyboard.

Listen to perfect fifth interval (C-G):

This interval is the same that constitutes the 5th chord. For example, C5 with the notes C and G or D5 with the notes D and F#.

To being able to distinguish this interval by ear, a good idea is to think about familiar songs which first notes match it. One such example is the children song "Twinkle, Twinkle, Little Star", that starts with an ascending P5 (in C Major: C-G).

Ascending perfect fifth intervals

A list with ascending perfect fifth intervals:

Inversions of these intervals will result in perfect fourth intervals.

Below is an example with ascending fifth intervals in pairs:
First piano note example with fifths

Descending perfect fifth intervals

A list with descending perfect fifth intervals:

Below is an example with descending fifth intervals in pairs:
First piano note example with fifths

See also the Circle of fifths.

Diminished fifth (tritone)

Diminished fifth in musical notationA related interval is the diminished fifth, or tritone (abbreviated TT), sometimes referred to the devil's interval.

It includes one semi-step less than the perfect fifth interval and makes up a half octave. For example, C to Gb. Because of the half octave distance, an inversion of a tritone will always be another tritone.

The sound of the diminished fifth is dissonant and the interval is used in few chords, but can be found in diminished chords. For example, Cdim which includes the notes C, Eb, Gb and Cdim7 which includes C, Eb, Gb, A.

Listen to tritone interval (C-Gb):

Keyboard diminished fifth interval 
A diminished fifth C to Gb on the keyboard.

Notice that the diminished fifth is identical with the augmented fourth.