How do bar lines divide music into measures?

In musical notation, bar lines are vertical lines that divide the staff into segments of time known as measures or bars.12

These lines serve two primary functions in organizing rhythm and timing:

• Indication of Stress: The bar line primarily marks a point of main stress, helping to establish an underlying regular pulse within the music.3
• Organizational Framework: Used in conjunction with a time signature, bar lines define the duration of each measure and clarify the grouping of beats.13

While bar lines are typically spaced equally to represent consistent durations, they are a relatively modern development in Western music, having been used to indicate time measures only since the 15th century. By periodically making one note stronger than others, these divisions allow the mind to perceive a structured succession of strong and weak beats.32