Editing Interval (music) (section)

=== Frequency ratios ===
{{Main|Interval ratio}}

The size of an interval between two notes may be measured by the [[ratio]] of their [[frequency|frequencies]]. When a [[musical instrument]] is tuned using a [[just intonation]] tuning system, the size of the main intervals can be expressed by small-[[Integer number|integer]] ratios, such as 1:1 ([[unison]]), 2:1 ([[octave]]), 5:3 ([[major sixth]]), 3:2 ([[perfect fifth]]), 4:3 ([[perfect fourth]]), 5:4 ([[major third]]), 6:5 ([[minor third]]). Intervals with small-integer ratios are often called ''just intervals'', or ''pure intervals''.

Most commonly, however, musical instruments are nowadays tuned using a different tuning system, called [[12-tone equal temperament]]. As a consequence, the size of most equal-tempered intervals cannot be expressed by small-integer ratios, although it is very close to the size of the corresponding just intervals. For instance, an [[equal temperament|equal-tempered]] fifth has a frequency ratio of 2{{sup|{{frac|7|12}}}}:1, approximately equal to 1.498:1, or 2.997:2 (very close to 3:2). For a comparison between the size of intervals in different tuning systems, see {{Section link||Size of intervals used in different tuning systems}}.