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Single and double vertical lines in lyrics notation are typically used to indicate the structure of the song and the timing of the lyrics.

Single vertical line (|)

A single vertical line is often used to indicate the end of a musical phrase or section. In some cases, it may also be used to indicate a change in the chord progression or rhythm.

Double vertical line (||)

 A double vertical line is typically used to indicate the end of a complete section or verse of the song. In some cases, it may also indicate the end of a chorus or bridge.

Repeat signs

Double vertical lines can also be used in combination with other symbols to indicate repeated sections of the song. For example, a section of the song may be marked with a double vertical line and a repeat sign (":||"), indicating that the section should be played twice before moving on to the next section.

Time signature changes 

Double vertical lines can also be used to indicate changes in the time signature of the song. In this case, the double vertical lines will be accompanied by a new time signature.