One of the conditions for a distance measure d to be a metric is that d(x, y) = d(y, x). If this condition is relaxed so that the distance between two points can be different depending on the direction of travel, then d is said to be a quasi-metric, and the space is said to be a quasi-metric space. Quasi-metrics are common in real life—the times taken to walk between two points can be quite different if there is a steep gradient between them, for example, and one-way systems in towns mean there is often a different route required to go from B to A than will take you from A to B.