INTERVAL, in music, is the difference of pitch between sounds in respect to height or depth, or the distance on the stave from one note to another, in opposition to the unison, which is two sounds exactly of the. same pitch. From the nature of our system of musical notation, which is on five lines and the four intervening spaces, and from the notes of the scale being named by the first seven letters of the alphabet, with repe titions in every octave, it follows that there can only be six different intervals in the natural diatonic scale until the octave of the unison be attained. To reckon from C upwards, we find the following intervals; thus C to D is a second; C to E is a third: C to F is a fourth; C to G, a fifth; C to A, a sixth; C to B, a seventh; and from C to U is the octave, or the beginning of a similar series. Intervals above the octave are therefore merely a repetition of those an octave lower; thus from C to D above the octave, although sometimes necessarily call a ninth, is neither more nor less than the same interval which, at an octave lower, is termed the second. A flat or a sharp placed before either of the notes of an interval does not alter the name of the interval, although it affects its quality; for example, from C to C4 is still a fifth, notwithstanding that the G is raised a semitone by the sharp. Intervals are classified as perfect, major, and
minor. Perfect intervals are those which admit of no change whatever without destroy ing their consonance; these are the fourth, fifth, and the octave. Intervals which admit of being raised or lowered a semitone, and are still consonant, are distinguished by the term major or minor, according as the distance between the notes of the interval is large or small, Such intervals are the third and sixth; for example, from C to E is a major third, the consonance being in the proportion of 5 to 4; when the E is lowered a semi tone by a fiat, the interval is still consonant, but in the proportion of 6 to 5, and is called a minor third. The same description applies to the interval of the sixth from U to A, and from C to Ab. The second and seventh, though reckoned as dissonances, are also distinguished as major and minor. The terms "extreme sharp" and "dimin ished" are applied to intervals when they are still further elevated or depressed by a sharp or a flat. For the mathematical proportions of intervals, see HAnmoNics.