Formed by the confluence of the Alleghany and the Monongahela, pursues a westward course of 1000 m., separating Ohio, Indiana, and Illinois from West Virginia and Kentucky, and after receiving sundry tributaries joins the Mississippi, being the largest and, next to the Missouri, the longest of its affluents; it is navigable for the whole of its course; on its banks stand Pittsburg, Cincinnati, Louisville, and Madison.