Association of numbers with physical quantities and natural phenomena by comparing an unknown quantity with a known quantity of the same kind. Weights and measures are standard quantities with which such comparisons are made. The earliest ones measured mass (weight), volume (liquid or dry measure), length, and area using units mostly based on dimensions of the human body. The cubit, representing the distance from elbow to fingertips, was the most widespread unit of measure in the ancient world. As such units were standardized, more were added, including units of temperature, luminosity, pressure, and electric current. Measurements made by the senses instead of by measurement devices are called estimates (seeestimation).