Interval

In music theory, an interval is a combination of two notes, or the ratio between their frequencies. Two-note combinations are also called dyads. Although chords are often defined as sets of three or more notes, intervals are sometimes considered to be the simplest kind of chord.

Intervals may be described as:
 * Vertical or harmonic, if the two notes sound simultaneously
 * Horizontal, linear, or melodic if they sound successively.

In Western culture, the most common method to classify and name intervals is based on their quality (perfect, major, minor, etc.) and number (unison, second, third, etc.). For instance, two frequently used types of interval are called minor third and major third. Intervals may be also classified as: Diatonic intervals, between the notes of a diatonic scale, or Chromatic intervals, non-diatonic intervals between the notes of a chromatic scale.