A bar, also called a measure, divides and organizes music into meter. A bar doesn't really mean anything until it has a meter, or time signature.
In a bar with the time signature of 4/4, the bar will contain 4 beats. In the video above this is demonstrated with a quick example from Twinkle, Twinkle Little Star.
Each bar is separated by a bar line. There are several types of bar lines but for the most part you will encounter the normal bar line.
At the end of every piece you will see a final bar line. This bar line consists of two lines, the left line is thin, and the right line is thick. This is also called a double bar line.
It is important to note that there are two kinds of double bar lines. One is the final bar line as we just mentioned and the other is a double bar line that uses two thin lines.
The double bar line with two thin lines is only used to separate large sections of music, but is never used to used to signify the end of a piece. This is a job saved only for the final bar line.
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