The Seventh and the Second

The seventh spans seven diatonic scale degrees, the minor seventh contains 10 semitones, the major seventh contains 11 semitones. The pitch ratio of the minor seventh is 9/5 or 7/4 and of the major seventh is 15/8.

The seventh are the first extensions of the diatonic chords:

The sevenths are considered dissonant intervals and they rarely occur at the beginning of the songs. In order to memorize the major seventh, you'll have to keep in mind that only the minor second sounds more dissonant. A classical example is Steve Howe's acoustic solo in The Ancients:

The minor seventh may be heard as a part of the dominant chord but it shouldn't be confused with the diminished fifth of the same chord when practicing. Maybe it's easier to hear a minor seventh as a major second lower than the octave of the first note.

The second spans two diatonic scale degrees, the minor second contains 1 semitone, the major second contains 2 semitones. The pitch ratio of the minor second is 16/15 or 25/24 and of the major second is 9/8 or 10/9. The minor second is also called a semitone and the major second is also called a tone.

They are dissonant intervals. The minor second sounds as the most dissonant interval of all.

There are countless tunes that begin with a second. It's easy to memorize the ascending major second at the beginning of the major scale and the ascending minor second at the end of the major scale: