A common security feature of many modern banknotes is a vertical thread which can be seen when the note is held up to the light. Thin metal threads were introduced on Bank of England notes in 1940, as protection against forgery during the Second World War (1939-45). At the same time, printing techniques were improved, and the colours of one pound and ten shilling notes were temporarily changed: the one pound from green to blue and pink, and the ten shilling note from reddish-brown to purple.Security threads are put in as part of the process of manufacturing the paper on which notes are printed. They are now so sophisticated that they may contain their own security devices, such as micro-printed text or magnetic data that can only be read by machine. Some become fluorescent under ultra-violet light. The thread may also be 'windowed', that is, woven through the paper to appear at intervals on the surface of the note.