It was originally based on three calibrating points. The first point was to allow a mixture of ice, water, and ammonium chloride to reach equilibrium and recorded the lowest temperature as 0 °F. The second point was recorded at 32 °F when ice would start to form on the surface of still water. The third point was assigned to 96 °F and taken from the body temperature of a person. This point was later changed the top number at 212 °F for the boiling point of water.