Drainage systems may be divided into two categories, surface and subsurface. Each has several components with similar functions but different names. At the lower, or disposal, end of either system is an outlet. In order of decreasing size, the components of a surface system are the main collection ditch, field ditch, and field drain; and for a subsurface system, main, submain, and lateral conduits from the submain. The outlet is the point of disposal of water from the system; the main carries water to the outlet; the submain or field ditch collects water from a number of smaller units and carries it to the main; and the lateral or field drain, the smallest unit of the system, removes the water from the soil. The outlet for a drainage system may be a natural stream or river or a large constructed ditch. A constructed ditch usually is trapezoidal in section with side banks flat enough to be stable. Grass may be grown on the banks, which are kept clear of trees and brush that would interfere with the flow of water. A surface drainage system removes water from the surface of the soil and to approximately the bottom of the field ditches. A surface system is the only means for drainage improvement on soils that transmit water slowly. Individual surface drains also are used to supplement subsurface systems by removing water from ponded areas.