Have you ever heard musicians explain the chords to a song with numbers? A guitar player might say something like, “This song is just I (one), IV (four), V (five) in G.” This number system applies to the chords in a given key (or family). Once this is understood it’s a great tool for learning and memorizing chord progressions for your favorite tunes. Plus it makes it easy to change the chords of a song into all the different keys.
Sometimes musicians use a numbering system to identify chords. Using numbers in place the letter names allows students to learn general rules about chords in all keys at once. That’s because all the rules (aka: Chord Theory) apply to chord groups in all twelve keys. So when you learn how to use a “ii (two), V (five), I (one)” in a key…you’re actually learning it for EVERY key.
Each number refers to the note in the scale that the chord is based on. For example, if I took the notes of the C Major scale and numbered them in order I get the following:
Each time the scale reaches an Octave it repeats the cycle. If we need to number notes beyond the octave, then we just continue counting past the seventh note:
* NOT common. These revert back to number used in first Octave.
By definition as chord is a group of three or more notes played together. Any three note group is called a Triad. Many of the most common chords we first learn are basic triads.
Lets build a chord from the first note of the scale: C (or the ‘1’). This note names the chord and is called the Root. Now we’ll add the other two notes by skipping every other one. This is referred to as stacked thirds. The result is a chord that contains the notes: C-E-G (or ‘1-3-5’). The C Major chord built from the 1-3-5 of the C Major scale.
Stacking thirds from any note of the scale will give some type of chord. The chord is named after the first note of the triad. Roman numerals are used when referring to the chord by number. Arabic numbers are used when referring to the note of the scale.
If we continue to stack thirds we get chords with more than three notes. The most common examples are the ‘seventh chords’ that we find in Jazz music. The same principles used for triads apply here, we just add an extra note.