machine for planting seed at a controlled depth and in specified amounts. The earliest known version, invented in Mesopotamia (Mesopotamia, history of) by 2000 BC, consisted of a wooden plow equipped with a seed hopper and a tube that conveyed the seed to the furrow. By the 17th century, metering systems were in use to ensure accuracy of the rate of planting; most consisted of wheels bearing small spoons that dipped into the seed hopper and guided it to the furrows in standard amounts.

Modern grain drills have a variety of metering systems and furrow openers. In general, the metering device—spoon, cup, fluted roll, or other—passes the seed by tube to one of several furrow openers, which are forced into the soil by springs or weights, with a short length of chain dragged behind to cover the seed. Drill widths are determined by the number and spacing of furrow openers.