The practical reasons for these bottom forms are not clear. For canoes used in rapid streams or in exposed waters where high winds were to be met many Indians preferred bottoms that were straight. Others in these same conditions preferred them rockered to varying degrees. It is possible that rocker may be desirable in canoes that must be run ashore end-on in surf. Of course, a strongly rockered bottom permits quick turning; this may have been appreciated by some tribal groups. Still other Indians appear to have believed that a canoe with a slightly rockered bottom could be paddled more easily than one having a perfectly straight bottom. The midsections of bark canoes varied somewhat in form within a single tribal type, because the method of construction did not give absolute control of the sectional shape during the building, but, on the whole, the shape followed tribal custom, being modified only to meet use requirements. Perhaps the two most common midsection shapes were the ~U~-form, with the bottom somewhat flattened, and the dish-shape, having rather straight, flaring sides combined with a narrow, flat, or nearly flat bottom. Some eastern canoes showed marked tumble-home in the topside above the bilge; often they had a wide and rather flat rounded bottom, with a short, hard turn in the bilge. A few eastern canoes, used mainly in open waters along the coast, had bottoms with deadrise--that is, a shallow ~V~-form, the apex of the ~V~ being much rounded; the ~V~-bottom, of course, would have aided in steering the canoe in strong winds. One type of canoe with this rising bottom had tumble-home topsides, but another, used under severe conditions, had a midsection that was an almost perfect ~V~, the apex being rounded but with so little curvature in the arms that no bilge could be seen. Generally speaking, the eastern canoes had a rather well rounded bottom with a high turn of the bilge and some tumble-home above, though they might have a flatter form when built for shallow-water use or for increased carrying capacity. A canoe built for speed, however, might be very round on the bottom, and it might or might not have some tumble-home in the topside. In the West, a flat bottom with flaring topsides predominated; fast canoes there had a very narrow, flat bottom with some flare, the width of the bottom and the amount of flare being increased to give greater capacity on a shallow draft. Some canoes in the Northwest had a skiff-form flat bottom and flaring sides, with the chine rounded off sharply.