How chords work

Three notes make a chord

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Three notes make a chord
Take a note, skip one, take the next, skip one, take the next: that is a triad, the chord almost all music is built from. On C it comes out C, E, G. Because you are skipping alternate steps, the shape looks like a snowman on the page — three noteheads stacked with no gaps.

The What the marks mean names the marks themselves, and the Little tools are there when you want to try something out.