Just perfect fifth
The just perfect fifth is the largest superparticular interval, spanning the distance between the 2nd and 3rd harmonics. It has a frequency ratio of 3:2, with a size of approximately 701.955 cents. It is an interval with low harmonic entropy, and therefore high consonance.
Variations of the fifth (whether just or not) appear in most music of the world. On a harmonic instrument, the third harmonic is usually the loudest which is not an octave double of the fundamental. Treatment of the perfect fifth as consonant historically precedes treatment of the major third (see 5:4) as consonant. 3:2 is the simple JI interval best approximated by 12edo, after the octave.
Producing a chain of just perfect fifths yields Pythagorean tuning. Such a chain does not close at a circle, but continues infinitely. 12edo is a system which flattens the perfect fifth by about 2 cents so that the circle close at 12 tones. Meanwhile, meantone temperaments are systems which flatten the perfect fifth such that the major third generated by four fifths upward be closer to 5:4 -- or, in the case of quarter-comma meantone (see 31edo), identical.
(see all at The On-Line Encyclopedia of Integer Sequences (OEIS); also relevant are the denominators of the convergents to log2(3)).
In composition, the presence of perfect fifths can provide a "ground" upon which unusual intervals may be placed while still sounding structurally coherent. Systems excluding perfect fifths can thus sound more "xenharmonic".
...see also Perfect fifth on Wikipedia.