The banner system was established by Nurhaci in the early seventeenth century. By 1601 Nurhaci was reorganizing his military forces into the basic structure of the banners and some evidence suggests that he might have started as much as a decade earlier. There are clear references to military units called "banners" in Korean sources in 1607 and sources dating from 1615 describe the "banner" unit structure. Details are uncertain due to the scarcity of source material and a lack of cultural referents; compounding the matter is a linguistic difficulty: In Manchu the term gūsa denotes a large military formation called a "banner" and tu refers to a flag known as a "banner", but in Chinese (the language used in nearly all the pertinent records) the character qi (旗) is used for both meanings. Thu