A chord is a number of notes
played simultaneously.
Chords are built from scales,
the root note of the scale/chord
is referred to as its key.

Each chord can be played in
one of 12 keys, as there are
12 possible root tones in the
chromatic scale.
The different types of chords are
constructed from the notes of
the chromatic scale
in various combinations.
The notes of a chord are
found by their interval,
or distance from the root note.

Chords are assembled according
to their interval pattern or formula.
From any particular root note,
the additional notes are
found by their intervals.
To form a chord,
apply the interval pattern to a
(diatonic) major scale starting at the
root note. This formula
tells you which notes from the scale
make up the chord.