Ask people to sing a piece of music and they will almost always sing its melody. It is the part we remember, whistle and get stuck in our heads. Yet melodies rarely stand alone, and how they are woven together with other lines is one of music's richest arts.

The shape of a melody

A melody is a series of pitches heard as a single, connected line. Its most basic feature is shape. A melody's contour is its overall pattern of rising and falling, the line you would draw if you traced its path with a finger. "Somewhere Over the Rainbow" opens with a huge rising leap and then gently falls; "Joy to the World" begins at the top and walks straight down the scale. Melodies that move mostly by small steps are called conjunct and tend to sound smooth and singable. Melodies with many large leaps are called disjunct and sound more dramatic or athletic. Most good tunes mix the two, often following a leap with steps in the opposite direction, which feels like a balance restored.

Melodies are grouped into phrases, musical sentences that end in a cadence. Many tunes pair phrases as question and answer: the first ends on an unsettled note, the second comes home.

Motives and sequences

Composers build long stretches of music out of very small ideas. A motive is a short, recognizable musical idea that recurs and develops through a piece. The most famous example is the opening of Beethoven's Fifth Symphony, first performed in 1808: three quick notes and a long one, short short short long. Beethoven repeats that motive, inverts it, shrinks it and passes it among the instruments until it saturates the whole first movement.

One simple way to develop a motive is the sequence. A sequence repeats a melodic idea starting on a different pitch, higher or lower, so the pattern climbs or descends like stairs. Sequences give music a feeling of purposeful motion, and you can hear them in everything from Vivaldi's concertos to pop choruses. In popular music, a short, catchy idea that keeps returning is often called a hook.

Texture: how many lines, and how they relate

Texture describes how many layers of sound are present and how they relate to one another. Three basic kinds cover most music. Monophony is a single melodic line with no accompaniment, whether sung by one person or by many in unison. Medieval Gregorian chant is the classic example, as is a person humming alone.

Homophony is a melody supported by chords. The melody leads and everything else moves along beneath it as backing. A singer with a strumming guitar, a hymn sung in four parts moving together, and most pop songs are homophonic.

Polyphony is two or more independent melodies sounding at the same time. Each line has its own shape and rhythm and could stand alone, yet together they make sense.

Counterpoint

The art of writing polyphony is called counterpoint, from the Latin for "point against point," meaning note against note. Good counterpoint balances independence and agreement: the lines should move differently, often in opposite directions, yet form pleasing harmonies where they meet. The simplest form is the round. In a round, voices sing the same melody but start at staggered times, so the tune overlaps itself. A more elaborate form is the fugue, which Johann Sebastian Bach brought to a peak. A fugue introduces a short melody called the subject in one voice, then has each other voice enter with the same subject in turn while the earlier voices continue with new material.

Call and response

Another way to relate lines is through dialogue. In call and response, a leader sings or plays a phrase and a group answers it. The practice is deeply rooted in many West African traditions, and it traveled with enslaved Africans to the Americas, where it shaped work songs, spirituals, gospel, the blues and jazz, in which instruments often answer a singer or one another. It is music built on participation: everyone has a part, and the community completes the song together.

Worked example: singing "Row, Row, Row Your Boat" as a round

Gather three people. The first starts singing "Row, Row, Row Your Boat." When that singer reaches the second line, "gently down the stream," the second person starts from the beginning. When the first singer reaches the third line, the third person begins. Each singer runs through the song twice and drops out.

Listen to what happens. At any moment three different parts of the same melody are sounding together, yet they fit. That is because nearly every line of the song fits over the same chord, the tonic of the key, so stacking the lines creates harmony rather than chaos. The steady "row, row, row" of one voice sits under the quick running "merrily, merrily" of another, so the rhythms also interlock instead of colliding. You have just performed polyphony: independent lines, each singable alone, combining into something fuller. A fugue by Bach uses the same principle with far more freedom, but the root idea is the one you find in a children's round.