It is used to measure the size of a wire and thickness of sheet.
The standard wire gauge is a circular metal disc with varying hole and slot size on its circumference. Each slot size corresponds to a gauge number which is written just below the hole.
The gauge numbers specify the size of a round wire in terms of its diameter.
As the gauge number increase from 0 to 36, the dia size decrease. The thickness of sheet metal and the diameter of wires confirm to various gauging numbers and the following Table 1 give the decimal equivalents of the different gauge numbers for the diameter of wires, and the thickness of sheets.