Related to the historical identification of mass and weight, the pound has been used both as a unit of mass and as a unit of force. In the United States, United Kingdom, and elsewhere, the pound is now officially defined as a unit of mass. The corresponding force is called a pound-force, and similarly the weight of a kilogram of material on Earth is called a kilogram-force. However the use of pounds to measure forces is still very common in physics and engineering, and in derivative units like p.s.i. (pounds per square inch). In most countries scientists have adopted SI units, which use kilograms for mass and newtons for force non-interchangeably.