A vertical line drawn through the staff to separate measures. Single bar lines divide measures; double bar lines mark section changes; a final bar line (thin + thick) marks the end.
Bar lines were introduced in the late 16th century. Before that, Renaissance polyphony was written without barlines — singers followed each other by ear.
Understanding bar line (measure line) is essential for reading and writing music accurately. Whether you are a beginner learning to read notation or an experienced composer, knowing how this musical element works improves your musicianship.
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