Major Second

Play C followed by D. The movement covers one whole step. It is wider than the one-semitone movement from C to D flat, but still small enough to function as ordinary stepwise motion.

Now play C and D together. The notes create more friction than a third or fifth, especially in a close register. The interval is common in melodies, while its harmonic form often works as color, suspension or controlled dissonance.

The labels neutral, open or tense depend on what surrounds the interval. The measurable part stays the same: two semitones and two neighboring letter names.

Count the letter names from C to D, including both ends: C, D. That makes it a second. The distance of two semitones makes it major.

C to D flat is also a second because it uses the letters C and D. It covers one semitone, so it is a minor second.

C to C double sharp would sound like C to D on an equal-tempered piano, but it repeats the letter C and is therefore an augmented unison. Interval names describe written relationships as well as keyboard distance.

The C major scale is C, D, E, F, G, A, B, C. It contains five major-second steps and two minor-second steps:

The order whole, whole, half, whole, whole, whole, half defines the major-scale pattern. Start that pattern on another note and the necessary sharps or flats preserve the same distances.

Other scales arrange whole and half steps differently. The whole-tone scale goes further and uses only major seconds. The chromatic scale uses only minor seconds. Saying that a scale moves by step tells you that adjacent scale degrees are used; the exact kind of step still depends on the scale.

C to D within one octave is a major second. C up to the D in the next octave is a major ninth. A ninth is a compound interval: an octave plus a second.

The two intervals use the same pitch classes, but their spacing changes the sound. C and D beside each other create obvious close friction. Put D an octave higher and the notes have more room, so the added tone can sound clearer above a chord.

Chord symbols often name the compound version. Cadd9 contains the C major triad C-E-G plus D. The D may be voiced near C or high above it, but its chord function is still called the ninth.

A Csus2 chord contains C, D and G. D replaces the third E, so the chord does not state C major or C minor in the usual way.

A Cadd9 chord contains C, E, G and D. The major third E remains, and D is added as another color. Both chords contain C and D, but their note collections and harmonic jobs differ.

This is why an interval name alone does not identify a chord. First find the root, then check which chord tones are present and which are missing.

Several chord types contain two notes a whole step apart:

Voicing determines whether that second is placed close together or spread into a ninth. Arrangers use the choice to control density without changing the underlying chord symbol.

In a melody, C and D happen at different times. Your ear follows a small step from one pitch to the next, so there is no sustained acoustic clash between them.

When C and D sound together, their nearby frequencies and partials interact. A piano voicing in the middle register makes that friction easy to hear. Move D an octave higher, shorten the notes or change the instrument and the texture becomes clearer.

Harmony adds another layer. D over a C major chord may sound spacious as an added ninth. D in Csus2 with no E leaves the major or minor quality open. The two-semitone distance stays fixed while timing, register and chord membership change its role.

In equal temperament a major second measures 200 cents. A pure whole tone can use ratios such as 9:8 or 10:9, depending on its harmonic context. Tuning and Temperament explains why fixed-key instruments use one equal size for all whole steps.