String distances that implement this interface are metrics. This means: - d(x, y) ≥ 0 (non-negativity, or separation axiom) - d(x, y) = 0 if and only if x = y (identity, or coincidence axiom) - d(x, y) = d(y, x) (symmetry) - d(x, z) ≤ d(x, y) + d(y, z) (triangle inequality).