Since Cauchy sequences can also be defined in general topological groups, an alternative to relying on a metric structure for defining completeness and constructing the completion of a space is to use a group structure. This is most often seen in the context of topological vector spaces, but requires only the existence of a continuous "subtraction" operation. In this setting, the distance between two points "x" and "y" is gauged not by a real number "ε" via the metric "d" in the comparison "d"("x", "y") < "ε", but by an open neighbourhood "N" of 0 via subtraction in the comparison "x" − "y" ∈ "N".