Bars/Measures and Bar lines
Composers and performers find it helpful to 'parcel up' groups of notes into bars, although this did not become prevalent until the seventeenth century. In the United States a bar is called by the old English name, measure, and bar is used for the UK English bar line. Each bar (or measure) contains a particular number of notes of a specified denomination and, all other things being equal, successive bars (measures) each have the same temporal duration. The number of notes of a particular denomination that make up one bar (measure) is indicated by the time signature. We will examine time signatures further in the lesson 4.
The end of each bar (measure) is marked usually with a single vertical line drawn from the top line to the bottom line of the staff or stave. This line is called a bar line (or bar).
As well as the single bar line (single bar), you may also meet two other kinds of bar line (bar).
The thin double bar line (double bar) (two thin lines) is used to mark sections within a piece of music. Sometimes, when the double bar line (double bar) is used to mark the beginning of a new section in the score, a letter or number may be placed above it.
The final double bar line (final double bar), also known as a period double bar line (period double bar) or terminal double bar line (terminal double bar) (a thin line followed by a thick line), is used to mark the very end of a piece of music or of a particular movement within it.
In music scored for keyboard instruments, where the music lies across two staves, the upper indicating the notes to be played by the right hand, the lower indicating the notes to be played by the left hand, bar lines (bars) are commonly drawn from the top of the upper line on the upper staff to the bottom line on the lower staff. This is illustrated below.
We thanks Chris Bee for clarifying the US terminology.