Of the geographical elements which go to form frontiers, the most obvious, after the sea, is constituted by mountains. The Pyrenees are a perfect example of a natural frontier which is also an actual frontier, and so in a lesser degree are the Alps and the Carpathians. Mountains inevitably divide, turning peoples different ways, in spite of modern means of communication, and they have always been valued as military barriers. Rivers, on the contrary, although they have military value, unite rather than divide, so that we need not be surprised if we find no important instances of a river frontier in present-day Europe, save along the Danube and where the Rhine separates Alsace and Baden. Most frontiers are neither mountain ranges nor rivers, yet they are often adjusted to lesser features of topography, with reference either to defence or to means of communication. Communication notably, with the growth of modern systems of transportation, bears an intimate relation to boundary problems. Access to the sea, either directly or by neutralized or internationalized rivers, has become a prime necessity for most states, and occupied the Conference especially in the cases of Poland and Czecho-Slovakia. Even railroad lines, especially where they monopolize natural routes, have their place in frontier adjustments, as in Carinthia or between East and West Prussia. Another geographical element, essentially modern in its significance, is found in natural resources. This has never been wholly absent from boundary problems, at least in its early form of fertile or less fertile land, but it has taken on a preponderant importance with the growth of modern industrialism. Each state has been anxious to bring within its limits supplies of mineral resources, and especially of that foundation of modern industry, coal. Now it so happens that some of the most important deposits of coal and mineral wealth lie on or near disputed frontiers. The coal of Upper Silesia, Teschen, Limburg, and the Saar, the iron of Briey and annexed Lorraine, the potash of Upper Alsace, the mercury mines of Carniola, are all cases in point. Prussia was affected by such considerations in drawing the frontiers of 1815 and 1871; other countries had learned the lesson by 1919.