The major second, also known as M2, is an interval that is constructed on the second natural. For instance C to D or F to G. It is a major interval because, opposed to the minor/diminished or augmented second, there are 2 semitones between the notes.

This is also why E to F is not a major second. Because between these notes is only 1 semitone: it is a minor second. E flat to F, or E to F sharp would however be a major.

For more complex intervals, also note that E to G flat would sound the same as E to F sharp if you play it independently but is also not a major second. In this case because there are 3 naturals involved, namely: E F and G. So any interval with E and G would be a minor third. The same would apply for D# to F.

History

The M2 interval originates mainly from being used in the major scale.

Sound

The M2 sound is somewhat unstable but very open. When playing the interval on its own it can sound a bit dissonant because of the instability. To hear a great example listen to Tommy Emmanuel playing his Beatles medley. In the intro he’s using the major second as a sort of rhythm pattern over the bassline.

Playing it in a chord within a major key but without a third, or using it as a ninth makes the sound very lush and open. On a minor chord however, it has a bit more tendency to want to move up to the minor third.

Usage

You can use the major 2nd in any way including in melodies in major and minor keys. It is certainly used a lot as a passing tone in melodies. For instance: do re mi.

Adding the interval to a chord gives it a unique character when leaving out the third. It would be called sus2 (or, suspended 2) because its tension asks for a release towards the major third.

Using the 2nd in a chord where there is also a third makes it an add2. More usually written and referred to as add9, or add nine. Where the add2 would be played under the third (can be quite dissonant) and the add9 should be played above the octave.