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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.
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).
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.).
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.
The time signature is explained in detail in a dedicated course.
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.
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.
Last update on 2018/12/21
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