The measure

Listen to a piece of music and tap your foot along. Without thinking, your foot finds a steady tap that keeps repeating at equal intervals, like a clock. That tap is the pulse, and each tap is a beat.

Blatter notes that the pulse is always present, even when the listener does not actually hear it, and that a performer has to be aware of it at every moment (Blatter 2007, part 1, art. 4).

Beats fall into groups

The pulse is rarely a shapeless queue. Beats gather into small, regular groups: in twos, in threes, in fours. Each group is a measure, also called a bar, and on the page a vertical line, the barline, separates one from the next.

Here is the same melody written twice. First without barlines:

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Then with them:

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The sound is identical. But the second is far easier to read, because the barlines cut the queue into pieces of four beats, and the eye sees the pieces instead of adding up value after value. That is Blatter's case for barlines: without them, a reader loses their place in a long run of durations (Blatter 2007, part 1, art. 4).

Count the beats in each measure: in the first, a quarter, two eighths, a quarter and a quarter, which make four; in the second, a half and two quarters, four again; in the third, a whole note, four once more.

One word, two meanings

Lacerda points out that the Portuguese word "compasso" means two things (Lacerda 1966, ch. VI): the division of music into small parts, the general idea, and each of those parts, as when one says the melody above has three of them. Med records the same double reading: the principle that organises the beats and, concretely, the space between two barlines (Med 1996, ch. XIX). English happens to keep them apart. The principle is meter; each part is a measure, or a bar.

How many beats fit in a measure

A measure is named by the number of beats it holds. With two beats, it is duple:

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With three, triple, the meter of the waltz:

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With four, quadruple, the most common of all:

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Count out loud as you listen: "one, two; one, two" in the first, "one, two, three" in the second, "one, two, three, four" in the third. The "one" always comes back at the start of each measure.

There are larger measures. Lacerda names the measure of five beats and the one of seven, and explains that each is usually felt as the sum of two smaller ones: five as three plus two, or two plus three; seven as four plus three, or three plus four (Lacerda 1966, ch. VI).

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Med goes further and names them all, from the measure of a single beat to the measure of ten; and he notes that the one-beat measure, disputed by some theorists, is quite common in twentieth-century composers (Med 1996, ch. XIX). Lacerda observes that duple, triple and quadruple used to be the usual choices, and that composers now use all of them freely.

A beat is not a note

In the examples above, nearly every beat has one note. That only makes the counting easy. In practice, one note can take several beats, and one beat can split into several notes (Lacerda 1966, ch. VI).

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In the first measure, the half note takes two beats. In the second, the first two beats each split into two eighth notes. In all three, the count still comes to four. The pulse does not change; what changes is how many notes fall on it.

Equal and unequal measures

In most music, every measure has the same number of beats. But that is not a rule. Lacerda already defines the measure as a division of music into parts "of equal or variable length" (Lacerda 1966, ch. VI), and Blatter likewise says it is very common for measures to be the same length, but not required.

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Here the first measure has three beats, the second four, the third three again.

Barlines

A barline is a vertical line that crosses the whole staff, from the fifth line to the first, neither shorter nor longer. No barline is written before the first measure: the staff starts straight away with the clef (Med 1996, ch. XIX).

Two other barlines mark special moments. The double barline, two thin lines, separates sections of the music. The final barline, a thin line and a thick one, ends it.

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Med lists where the double barline appears: at the end of a section, at the division of a period, and at a change of key, of meter or of tempo. At the very end, the second line thickens.

What comes next

Counting "one, two, three" assumes the "one" stands out. Why the first beat sounds stronger than the rest is the subject of metric accent. And which note value takes one beat, and whether that beat divides into two or three, is what separates simple meters from compound ones, and what the time signature writes at the start of the staff.