A single chord is a color; a series of chords is a story. The order in which chords follow one another shapes whether music feels as if it is heading somewhere, pausing, finishing or being surprised.

Progressions

A chord progression is a series of chords played one after another. Some progressions are so common that they work like stock phrases. The motion from IV to V to I, which you met in "Happy Birthday," underlies centuries of hymns, folk songs and early rock and roll. The speed at which chords change is called the harmonic rhythm. In a slow ballad a chord may last a whole measure or more; in a busy jazz tune chords may change on nearly every beat.

One modern loop deserves special mention. The progression I, V, vi, IV (in C major: C, G, A minor, F) turns up in a remarkable number of pop songs, including the Beatles' "Let It Be."

Another influential pattern is the repeating chain of eight chords in Johann Pachelbel's Canon in D, probably written around 1700 and wildly popular again since the 1970s, which many later songwriters have borrowed.

The circle of fifths

Musicians often arrange the twelve keys in a ring called the circle of fifths. In the circle of fifths, each key sits a fifth away from its neighbors, and neighboring keys share all but one note. C major has no sharps; one step around to G adds one sharp; one more step to D adds a second; and so on, while going the other way adds flats. The circle is a map. It shows which keys are close relatives, and it explains why progressions whose roots fall by fifths, such as moving from D to G to C, sound so natural: each chord seems to lead into the next like a domino.

Cadences: harmonic punctuation

Music is grouped into phrases, much like sentences. A cadence is a chord pattern that ends a phrase, like punctuation at the end of a sentence. Western music uses a small set of them again and again.

An authentic cadence moves from V to I, dominant to tonic. It is the strongest close, like a period, and most classical pieces end with one, often repeated for emphasis. A plagal cadence moves from IV to I. It is gentler and is often called the "Amen" cadence, because many hymns end with it on the word amen.

A half cadence ends a phrase on V, leaving the tension unresolved. It works like a comma or a question mark: the listener knows more is coming. A deceptive cadence moves from V to vi instead of to I. The ear expects home and gets a minor chord that shares two of its notes. The effect is a gentle surprise, as if a door you expected to open onto your living room opened onto a garden.

Changing key

Longer pieces rarely stay in one key throughout. Modulation is changing from one key to another within a piece. A classical symphony movement may travel through several keys before returning home, and the return is part of the pleasure. Pop songs sometimes modulate up by a half or whole step near the end for a final chorus, a move so common that some musicians tease it as the "truck driver's gear change."

The twelve-bar blues

One progression has had a uniquely broad influence. The twelve-bar blues is a progression twelve measures long built from the I, IV and V chords. It took shape among Black Americans in the Deep South around the turn of the twentieth century, growing out of work songs, spirituals and field hollers, the musical expression of people who had been enslaved and were still denied basic rights. From that pattern grew rhythm and blues, rock and roll, and a large share of jazz. The blues also bends the scale: singers and guitarists slide certain notes slightly flat, the so-called blue notes, which fall between the Western major and minor and carry much of the style's expressive weight.

Worked example: mapping a twelve-bar blues

Picture twelve boxes in three rows of four, each box one measure of four beats. In the standard pattern, the first row holds four measures of I. The second row holds two measures of IV and then two of I. The third row holds one measure of V, one of IV, and two of I. In the key of E, a guitarist's favorite blues key, that means E for four bars, A for two, E for two, then B, A, E, E.

Blues lyrics often fit the rows: a first line in row one, the same line repeated over the IV chord in row two, and a different answering line in row three, so the third row resolves both the words and the harmony. Notice how the B in bar nine, the dominant, creates the moment of greatest tension, then steps down through A to home. Many players also slip in a B at the very end, making a half cadence that sends the music around for another chorus. Once you can feel those twelve bars, you can follow thousands of recordings.