Octaves, fifths, and fourths (the first four tones of the harmonic series at frequency ratios of 1:2, 2:3, and 3:4) are known as perfect intervals.
Seconds, thirds, sixths, and sevenths, however, have a major version and a minor version. In all cases, the major interval is one semitone higher than the minor interval.
A minor second is a second with one semitone.
A major second is a second with one whole tone.
A minor third is a one whole tone plus one semitone.
A major third is two whole tones.
A minor sixth is a perfect fifth plus one semitone.
A major sixth is a perfect fifth plus one whole tone.
A minor seventh is one whole tone below an octave.
A major seventh is one semitone below an octave.
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