Bar-lines are vertical lines drawn across the five lines of the staff. They serve an essential purpose in music notation by indicating the time and rhythm of the music. By dividing the music into equal portions, bar-lines create distinct sections known as bars or measures.

Accent refers to the emphasis or stress placed on certain notes in music at regular intervals of time. Accented notes typically occur immediately after each bar-line, specifically on the first beat of each bar. For instance, in the melody of the “National Anthem,” the accents fall on the first beat of each group of three beats, visually represented by the placement of bar-lines.
Without accent and bars music will look confusing like this

The concept of time in music relies on the regular recurrence of accented notes. In the aforementioned example, accents occur in groups of three beats, resulting in three beats per bar. Similarly, if accented notes occur in groups of two beats, there will be two beats in each bar. When the accent falls on groups of four beats, it corresponds to a time signature with four beats in each bar.

There are three primary types of time signatures: duple, triple, and quadruple (or common time). Each represents a specific number of beats per bar.
- The Time which has two beats in each bar is called Duple.
- The Time which has three beats in each bar is called Triple.
- Â Â Â Â Â Â The Time which has four beats in each bar is called Quadruple (or Common).