How are triads represented in sheet music? What are chord progressions?

A triad can be summed up by a single symbol, such as a lead-sheet chord symbol. A lead sheet symbol includes information about both root quality, as well as which pitch class occurs in the lowest voice (called the bass regardless of who is singing or playing that pitch).

In tonal music, chord progressions have the function of establishing or contradicting a tonality, the technical name for what is commonly understood as the "key" of a song or piece.

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Idk but maybe a camera?

They needed different and opinions