What is a 7 chord? 4 types explained - Major, minor, Dominant, Diminished

What is a 7 chord? What are they used for? There are four types of 7 chords, and assumptions that are made when we see or hear "7" next to a chord.
This is an instructional snip it of my Bluegrass, Country, and Roots School
AustinScelzo.com/bluegrass

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Like the rationale, the pause, and the overall presentation, as if; let's THINK this through together, at a leisure pace that helps understanding, and retention!