The first screw was made around 250 BC. Fasteners were found in ancient Egypt. In the middle ages, screws and nails were used in clocks, armor and furniture. Craftsmen had to always keep a nut, physically, on the bolt, because they would not fit similar looking parts made by other mechanics. Fasteners used to be extremely hard to replace and interchange. Each fastener was custom made; the threads did not mate, nor the head and drives match repeatedly. There were no real standards on fasteners until 1841. During WWII, USA, Britain, and Canada created the unified thread standard (inches) in 1949, so the countries could trade parts to fix equipment on the battle fields and at sea. The metric standard was created in 1960 (ISO). This allowed for replacement parts to become readily available and increase the speed of production. Now, fasteners are easily available online and are interchangeable because of these standards and a long history of trials.