Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In metric geometry, the Karlsruhe metric (the name alludes to the layout of the city of Karlsruhe) is a measure of distance that assumes travel is only possible along radial streets and along circular avenues around the center. The metric space which most closely corresponds to our intuitive understanding of space is the 3-dimensional Euclidean space. In fact, the notion of "metric" is a generalization of the Euclidean metric arising from the four long-known properties of the Euclidean distance. The Euclidean metric defines the distance between two points as the length of the straight line connecting them.