Bar Lines
A bar line is a vertical line drawn through a staff. The spaces between the bar lines are the measures defined by the time signature.
The vertical line at the left end of each line of music is NOT a bar line. It connects the staves that are to be played together in a musical score.
A bar line ONLY crosses between staves when the staves are played by the same instrument as in this typical piano-vocal score. The voice uses the top staff, the piano uses the bottom two.

Counting Measures
20 bars = 20 measures!
The first numbered measure is the first FULL measure. If there is an incomplete measure before the first measure, the missing beats are made up in the last measure, which IS counted!
If a measure number or a rehearsal letter appears above a bar it refers to the following measure. Here letter A is at measure 2.

Meters with more than 3 beats have an imaginary bar
line in the middle to make the music easier to read.
The top example could be written with a quarter note in the middle but the two-eighths are tied to show where the third beat is. Also the eighths are not barred across here.

In 6/8 meter, the imaginary bar line is between the third and fourth beats.

In 5/8 the position may change according to the needs of the music.

In complicated rhythms an imaginary bar line may be employed between every beat as in the bottom example.

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