Easy Harmonizing with Primary Triads

In tonal music, a triad is a group of three notes formed by stacking two thirds above a "root".  A triad can be formed above every note in a major or minor scale.  

For our purposes, the thirds stacked above each scale degree consist of notes that are within the same key as the scale.  The resulting chords are labeled according to the scale degree upon which they are built.  Roman numerals are used for this.  In major keys, the chords are labeled I, ii, iii, IV, V, vi, and vii˚.  Uppercase Roman numerals represent major chords, while lowercase Roman numerals indicate minor chords.  (The chord with the degree symbol is neither major, nor minor. It is a diminished chord.)

Just about every diatonic melody--a melody made up of notes that belong to a single key--can be harmonized using the three "primary" triads which are: I, IV, and V.  These chords are called "primary" chords because they express their functions clearly; hearing them quickly establishes what key a piece of music is in.  The chord built on the first scale degree is called the tonic.  The chord built on the fourth scale degree is called the subdominant, and the chord built on the fifth scale degree is called the dominant.

Every note of a diatonic scale can be found in one or two of these fundamental chords.  For example, in the key of C major, the C triad contains the notes C, E, and G.  All of these notes are found in the I chord of that key, so if we want to harmonize a C melody note, it can be harmonized with a C (or I) chord.  The IV chord contains notes F, A, and C, so a C melody note can also be harmonized with an F chord.  The V chord, G major--G, A, B--does not contain a C; therefore, a C melody note cannot be harmonized with a G chord.  So, we have two chord choices when harmonizing a C melody note: I and IV--C major and F major.  The chart below shows the primary chords available when harmonizing a given melody note in a C major melody.

C: I, IV

D: V  

E: I

F: IV

G: I, V

A: IV

B: V

Notice how easy harmonization is if we limit ourselves to these three chords.  Only C and G melody notes have two chord options. All other notes are harmonized by one chord, only!  

Let's practice!  Try harmonizing the first part of Twinkle, Twinkle Little Star (in C major) using only I, IV, and V chords (C, F, and G).  Here's the tune: C C G G A A G F F E E D D C

How did you do?  Now that you're getting the hang of this, try harmonizing other popular tunes with the three primary triads.  Happy harmonizing!