This is a wood which has probably been used more than any other kind in all classes of structures. In ancient times it was about the only wood in use both for the building of houses and for shipbuilding. Since the softer woods have become popular and oak has become somewhat less easy to get, its use has diminished to some extent, but it is still one of the most useful of woods. The trees grow freely all over the northern parts of Europe and America, extending as far south as the Equator, and have been particularly plentiful in the British Isles. There are about twenty different kinds of oaks to be found in various parts of the United States and Canada, but there are three distinctly different species, which are sold separately. These are the "white oak," the "red oak," and the "live oak." The red oak is usually more porous, less durable, and of coarser texture than the white oak or the live oak. The trees are of medium size and form a large proportion of all the broad-leaved forests. Live oak was once very extensively used, but has become scarce and is now expensive. Both the red oak and the white oak are used for inside finishing, but they are liable to shrink and crack and must, therefore, be thoroughly seasoned. They are of slightly different color, the white oak having a straw color while the red oak has a reddish tinge, so that they can not be used together where the work is to be finished by polishing. Oak is always best if quarter-sawed and it then shows what is known as the "silver grain." This is the result of the cutting of the medullary rays, and appears on the finished wood as a succession of splashes or blotches which are of lighter color than the rest of the wood and which glisten in the light.