In manufacturing and mechanical engineering, flatness is an important geometric condition for workpieces and tools.
In the manufacture of precision parts and assemblies, especially where parts will be required to be connected across a surface area in an air-tight or liquid-tight manner, flatness is a critical quality of the manufactured surfaces.
Metrology of such a surface can confirm and ensure that the required degree of flatness has been achieved as a key step in a manufacturing processes.
Two parts that are flat to about 1 helium light band (HLB) can be "wrung" together, which means they will cling to each other when placed in contact.
Sir Joseph Whitworth popularized the first practical method of making accurate flat surfaces during the 1830s,[2] using engineer's blue and scraping techniques on three trial surfaces, in what is known as Whitworth's three plates method.