In addition to horizontal staff lines, music — including piano music — employs some vertical lines to help you keep track of where you are in the music, sort of like punctuation in a written sentence.
Think of a music staff as a time line. In the same way that the face of a clock can be divided into minutes and seconds, the music staff can be divided into smaller units of time. These smaller units of time help you count the beat and know where you are in the song at all times.
A bar line divides music into measures (also called bars), breaking up the musical paragraph into smaller, measurable groups of notes and rests as shown in the following figure where the slash marks represent each beat:
Each measure has a specific number of beats — most commonly, four beats. Measures help group beats into patterns and help organize the writing and reading of music for both the composer and the performer. This smaller grouping of four beats is pretty easy to count: Just think “1, 2, 3, 4,” and then begin again with “1” in each subsequent measure.
A short, three-minute song can have more than 200 separate beats. Measures help make sure that you don’t get lost keeping time.
Five types of bar lines give you directions on how the music is structured, when and where to repeat, and when to stop:
Single: Go on to the next measure.
Double: Proceed to the next section (because you’ve reached the end of this one!).
Start repeat: Repeat back to this measure.
End repeat: Repeat back to the measure that begins with a start repeat (or to the beginning if you don’t see a start repeat).
Final: You’ve reached the end! Stop playing!
The five types of bar line.