The old system of English measurements such as the pound are based on convenient objects in the natural world. The "grain" is the smallest of these objects, and originally it meant the weight of a grain of wheat or barley. Each individual grain might be slightly larger or smaller than the next, but the more grains were added together to make a larger measure, the more these little differences would even out. Even so, these measures were not entirely precise and did not relate to other measures such as the inch or the foot.