bar lines and measure (or bar)

A measure of 4 quarter-notes to the bar.

A bar, also known as a measure, a length of time defined by a given number of beats of a given duration. Typically, a piece of music consists of a number of bars of the same length, and in modern musical notation the number of beats in each bar is specified at the beginning of the score by the top number of a time signature (such as 3/4). The use of bars and bar lines started in the sixteenth and seventeenth centuries, initially for writing choral music. See also pickup notes and measures.