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The first chord to learn when you start playing chords, is the root position three-note chord. It is also called a triad since it has only three notes; a root, a third and a fifth.
Basic chords are built with thirds stacked on top of each other. Learning to play triads (which are the basic three note chords) is the first step to know when learning chord piano. It is also very easy!
Here is a C major chord (triad) in root position:
Root position means that the chord is in its basic position with all the notes a third apart (skipping one white key between them). The bottom, or lowest note then gives its name to the chord.
(You could play the notes of the chord in another order, then the chord is inverted and not in the root position.)
Since C is the bottom note in the example above it is called a C chord. Here is what the numbers mean: