A bar line (vinculum) is a horizontal line used in mathematical notation for a specific purpose. It may be placed as an overline (or underline) over (or under) a mathematical expression to indicate that the expression is to be considered grouped together. Historically, vincula were extensively used to group items together, especially in written mathematics, but in modern mathematics this function has almost entirely been replaced by the use of parentheses.[1] Today, however, the common usage of a vinculum to indicate the repetend of a repeating decimal[2][3] is a significant exception and reflects the original usage.