They are used to seperate the divisions of time. The spaces in between the barlines are measures.
A barline is a line separating each measure in a piece of Western music.
Barline
I'm not sure what you mean by "barline headline." Barlines are vertical lines in sheet music that divide it into measures, while headlines are titles or headings at the top of a page. If you could provide more context, I may be able to better assist you.
It's called a double barline.
It happens right after the barline, in every measure
The repetition sign, denoted by a double barline, is used in sheet music to indicate that a section of music should be repeated. It typically involves going back to the beginning of the repeated section when encountered in the music score.
Double barline
To "tie" the beats of two notes together; if there was no tie then it would be impossible to carry a note through multiple measures or over a barline.
A type of a bar lines which single line that divides the staff into measure
Bar lines are placed on the musical staff to separate the divisions of time. Time is divided in groups of pulses as defined in the time signature.
A double bar - a thin line followed by a thick line - signifies the end of a piece of music. On occasion the word "Fine" (fee-nay) accompanies this symbol.
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