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The bar (or measure) structures a musical work in equal time units.

Bars and bar lines

A musical work is divided into several bars. Each bar of a piece has the same number of beats. The bars are separated by bar lines.

Bar lines

The bar line is a vertical line which crosses the five staff lines. The last bar of the piece is indicated by a final bar (a thin line followed by a thick line).

Bars and bar lines
FIGURE 1 - Ah! vous dirai-je maman

The bar lines are green. Between two bar lines, there is a bar (or measure). At the end of the song is the final bar line (in orange). In all, this piece contains 8 bars.

The time signature  indicates that each bar has 2 beats and that the value of one beat is equal to the quarter note.

Later in this course, a paragraph is devoted to the introduction on the time signature

If the work has several instruments and/or several staves per instrument, the vertical line crosses all staves of the same instrumental section (section of wind instruments, string instruments, etc.).

Bar lines crossing a staff system
In the following figure, the bar lines cross the entire section of the string instruments (from Violin I to Cello). Only the Violin solo is apart (1st staff up, in orange), its bar lines are not connected to the other staves to better distinguish it from the other instruments. Nevertheless, the bar lines are vertically aligned.

FIGURE 2 - The four seasons: Spring - Antonio Vivaldi

Time signature

In order to indicate the number of beats in a bar, we write at the beginning of the work the time signature (also called "meter signature" or "measure signature"). Time signature is formed by two numbers, one on the other — it is a kind of fraction, but without a horizontal line — and is placed right after the key signature (or the clef if there is no key signature), just before the first note of the song.

Time signature position

FIGURE 3 - Time signature position without any key signature

FIGURE 4 - Time signature position with a key signature

Unlike the key signature that is indicated at each new staff, the time signature is only indicated at the beginning of the work. It may happen, however, that a composer wishes to change the time signature during the song, in which case a new time signature is indicated.

The time signature is explained in detail in a dedicated course.

Measure numbering

In order to better find your way in a score, especially in a rehearsal context with other musicians, the measures are numbered. However, to avoid overloading the score, only the numbers of the measures at the beginning of the staff are written, except for the first measure of the song.

The bar numbers are placed above the staff, at the beginning of the bar.

Measure numbering

FIGURE 5 - Prelude to the Suite No. 1 for Cello - J.-S. Bach

Anacrusis (Pickup measure)

Sometimes the first measure of a piece is incomplete: this is called an anacrusis or a pickup measure. The song starts with an upbeat, before the first beat of the first complete measure.

This is the case in the excerpt of Vivaldi's Four seasons (Figures 2 and 4). The first measure contains only a single eighth while the time signature indicates 4 beats (). The eighth value is half a beat, it misses 3 beats and a half to make a complete measure: it is therefore an anacrusis.

The anacrusis is not taken into account in the measure numbering. In case of anacrusis, the measure No. 1 is therefore the first complete measure, that is to say the one following the anacrusis.

Last update on 2021/05/07

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