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. Each bar contains a particular number of notes of a specified denomination and, all other things being equal, successive bars each have the same temporal duration. The number of notes of a particular denomination that make up one bar is indicated by the time signature. We will examine time signatures further in the lesson 4. The end of each bar 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. As well as the single bar line, you may also meet two other kinds of bar line. The thin double bar line (two thin lines) is used to mark sections within a piece of music. Sometimes, when the double bar line is used to mark the beginning of a new section in the score, a letter or number may be placed above it. The double bar line (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 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. Bar/Measure Numbers and Letters :: It is common practice, when a piece of music is extended, to number the bars either at the beginning of each line or periodically - for example, by marking every fifth or every tenth bar. In this way, a group of musicians, performing from individual parts, can easily start from points within the piece of music by going from a particular bar. You will see, from the first example below, that the bar number is placed at the beginning of the bar just above the bar line that marks the end of the previous bar. If only marking the first bar of every line, as in the second example, the bar number is placed at the very beginning of the staff. From the nineteenth century rehearsal letters began to appear in scores to mark the beginnings of sections within the work. A conductor wishing to take an orchestra from, say, the beginning of the third section, which is marked in the score with a letter B, would ask the orchestra to 'go from letter B'. Bars/Measures and Ties :: Notes can be sustained over bar lines by linking them with ties. This is shown in the example below.