corn head is equipped with snap rolls that strip the stalk and leaf away from the ear, so that only the ear (and husk) enter the throat. This improves efficiency dramatically since so much less material must go through the cylinder. The corn head can be recognized by the presence of points between each row. Occasionally rowcrop heads are seen that function like a grain platform but have points between rows like a corn head. These are used to reduce the amount of weed seed picked up when harvesting small grains. Self-propelled Gleaner combines could be fitted with special tracks instead of tires to assist in harvesting rice. These tracks can be made to fit other combines by adding adapter plates. Some combines, particularly the pull type, have tires with a deep diamond tread which prevents sinking in mud. Conventional combine The cut crop is carried up the feeder throat (commonly called the "feederhouse"), by a chain and flight elevator, then fed into the threshing mechanism of the combine, consisting of a rotating threshing drum (commonly called the "cylinder"), to which grooved steel bars (rasp bars) are bolted. The rasp bars thresh or separate the grains and chaff from the straw through the action of the cylinder against the concave, a shaped "half drum", also fitted with steel bars and a meshed grill, through which grain, chaff and smaller debris may fall, whereas the straw, being too long, is carried through onto the straw walkers. This action is also allowed because grain is heavier than straw, which causes it to fall rather than "float" across from the cylinder/concave to the walkers. The drum speed is variably adjustable on most machines, whilst the distance between the drum and concave is finely adjustable fore, aft and together, to achieve optimum separation and output. Manually engaged disawning plates are usually fitted to the concave. These provide extra friction to remove the awns from barley crops. After the primary separation at the cylinder, the clean grain falls through the concave and to the shoe, which contains the chaffer and sieves. The shoe is common to both conventional combines and rotary combines. Hillside leveling In the Palouse region of the Pacific Northwest of the United States the combine is retrofitted with a hydraulic hillside leveling system. This allows the combine to harvest the steep but fertile soil in the region. Hillsides can be as steep