The D7 chord, or to give its full name, the D Dominant 7th chord

Dominant 7th chord

Dominant seventh chords are often built on the fifth scale degree (or dominant) of a key. For instance, in the C major scale, G is the fifth note of the scale, and the seventh chord built on G is the dominant seventh chord, G⁷ (shown above).

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, is a variation on the regular D chord. This means it has the same root position as D Major (the 'happy-sounding' D chord), which as you have probably guessed, is the note 'D' [1].

What does the 7 mean in a D7 chord?

A seventh chord is a chord consisting of a triad plus a note forming an interval of a seventh above the chord's root.

Why is it called a D7 chord piano?

D7 is what is called a “dominant 7th chord”. It is based on a major triad, but adds a minor 7th note to create the dominant 7th chord. This creates a very classy and elegant sound, that is neither major nor minor sounding, but actually both at the same time.