Rhumb lines gave a rational order to the mysterious depths. From the Spanish rumbo for a defined path, the rhumb line directed mariners along a constant cardinal direction. Guided by a rhumb line, a captain could set a course using just a compass, which was crucial when measuring longitude at sea was almost impossible. Rhumb lines were so important for so long that by the eighteenth century they were simply part of how many maps depicted water. For centuries, maps that were never made to go to sea or even to plan sea routes still incorporated rhumb lines because readers would be confused without them.