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Pretty much what the title says, why is it that chords like C7 or F7 have a minor seventh instead of a major one? Is there any particular reason to it?

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By definition. C7, F7, E♭7, etc. are "dominant seventh" chords, which have a minor seventh. A "major seventh" chord is notated differently: CM7, Cmaj7, or CΔ7.

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Some think that C7 (for example) belongs to key C. It doesn't. It belongs to (comes from, is diatonic in) key F It's the dominant chord in that key. As such, by 'stacking thirds', the interval between 1 and 7 (C and B♭) is m7.

In actual key C, the 'stacked thirds' give the interval between 1 and 7 (C and B) as M7.

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