Play a C Major scale on a piano or keyboard. Name each note by letter and number. The first note, C, is 1. The next note, D, is 2. The next note, E, is 3. This continues to B, the seventh note of the scale. The next note, C, is the octave of the first note and can have either 8 or 1 for a number. Notes in the upper and lower octaves repeat the scale with the same letters and numbers; in theory they are the same scale notes. Intervals in scale degrees are determined by numbers that are given to each note. For example, the E, No. 3, is an interval of a third above C, No. 1; G, No. 5, is an interval of a fifth above C.
Play a C, E, and G on a keyboard all at the same time. You have just played a chord. This three-note chord is called a triad. All triads are made up of a root, third and fifth. The root of the triad just played is C, the third is E and the fifth is G. Since chords are named after the root, the C-E-G triad is a C chord. Most of the time the chord root is positioned lower in the staff than the third or the fifth. However, it is common to see chords inverted. In the first inversion, the third is the lowest note; in the second inversion, the fifth is the lowest note.
Play a C, E, and G again but this time play one note at a time. You have just played a C arpeggio. It's common to see chords written and to hear them played in arpeggio form. Many arpeggios are played in more than one octave. Frequently, chords in arpeggio form are outlined in melodies, for example, the first phrase in the National Anthem, "Oh, say can you see?" outlines a triad in arpeggio form: 5-3-1-3-5-8. Play this arpeggio on the keyboard.
Play these triads in the following order: C-E-G, F-A-C, G-B-D, and C-E-G. You have just played a I, IV, V, I chord progression in the key of C. Chords are designated by Roman numerals based on the scale degree numbers. Now, play the G-B-D triad again but this time add an F to the chord. The result is a four-note seventh chord, or G7, so called because the added note, F, is an interval of a seventh above the root note, G. The progression of the V7 to I firmly establishes C as the key note and is widely used in songwriting harmony.