The I, IV, and V⁷ Chords.

The word chord refers to multiple notes being played at the same time. It is generally not called a "chord" until at least 3 different notes are played together, but sometimes 2 notes can be called a chord as well. 

There are a few basic chords that are very common in music. We could identify these by letter name, or we could identify these by how they work functionally within a key based on their scale degree. We label these using a system of Roman numerals and exponents.

Here are the three most common chords that are generally used in a major key:

1. The I chord.
   1. Also known as the "root" or the "tonic" chord. This is the most stable chord in a key and is considered to be at rest.
   2. It consists of the root, the major third, and the fifth.
   3. Example: In the key of C Major, the C Major chord is the I chord. It consists of the notes C, E, and G.
2. The IV chord.
   1. Also known as the "subdominant." This serves what we would call a "predominant" function, which means it is not the chord in the key with the most tension, but helps lead to this tension when desired.
   2. It consists of the fourth, the sixth, and the eighth (or root).
   3. Example: In the key of C Major, the F Major chord is the IV chord. It consists of the notes F, A, and C.
3. The V⁷ chord.
   1. Also known as the "dominant seventh." This is a 4-note chord, unlike the other two (which are 3-note chords), as the fourth note helps add maximal tension, that resolves completely when followed by the I chord.
   2. It consists of the fifth, the seventh, the ninth (or second), and the eleventh (or fourth).
   3. Example: In the key of C Major, the G dominant seventh consists of the notes G, B, D, and F.