Find the key signature of a song you are learning or want to compose in. Knowing the scale and key signature is essential for building chords because chords are built from scales. When reading from a sheet music score such as a lead sheet, look at the last note or chord on the music. The last chord or note of a song often indicates the key signature.
Build three-note chords on all the scale degrees. A scale consist of seven degrees. For example, the notes of C major are C, D, E, F, G, A and B. Construct a three-note chord, also called a triad, on every note of your scale. Each of the three notes should be a third apart from the previous note. In C major, the first chord includes C, E and G. Make sure every note of each chord lies within the key signature.
Add the seventh degree to the five chord. For example, the five chord in C major is the G chord, which consists of the notes G, B and D. The seventh degree of the five chord is F, which is also added forming G, B, D and F. The five, or dominant, chord will always contain the seventh degree even when adding other extensions or alterations.
Add extensions to the chords once you have found all the triads that occur naturally within the key signature. A chord extension refers to adding the ninth to eleventh or thirteenth degree of the diatonic scale to an existing triad. Adding extensions to chords provides a richer tonal color and enhances the mood of the composition. To find an extension, count a ninth, eleventh or thirteenth step above the tonic and add that interval to the existing triad. When playing an extended chord, the only necessary notes are the notes of the basic triad, the seventh and the extended tone.
Experiment with adding alterations to the chords in the key signature. An altered chord is a chord that does not exist naturally in the key signature. You can alter a chord in several ways. Find the ninth, eleventh or thirteenth degree of the scale and raise or lower that degree of the scale. Add the altered degree of the scale to an existing triad. The fifth degree of the scale can also be raised or lowered for an augmented or diminished chord.