A 7th chord is built from the root, third (or flatted third), fifth, and 7th (or flatted seventh).
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The other three tones from a major scale (2nd, 4th, 6th) can be added to the 7th chord. Since these notes are not replacing 7th chord tones – but are instead added on top – we refer to them as the 9th, 11th, and 13th.
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Diatonic scale in C with extensions: C (Root) D (2nd) E (3rd) F (4th) G (5th) A (6th) B (7th) C (Root) D (9th) F (11th) G (13th)
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These three tones can be added to Maj7, min7, and dominant 7th chords in order to add different sonic colors.