Japan borrowed the Chinese system of writing, including its numerals and calculating methods, in the eighth century CE. While the Chinese writing was adapted to write Japanese, the borrowed numerals retained their Chinese forms and names, displacing but not replacing the earlier Native Japanese numbers. The Native Japanese numbers were formed by doubling, and scholars have speculated that the system of doubling or pairing may have been related to ancient finger-counting. However, the finger-counting found in Japan today bears no trace of pairing, though paired finger-counting is found in other parts of the world (for example, in Africa, where it takes the form of finger-counting that is symmetrical rather than sequential). Nor did the Japanese adopt the Chinese method of finger-counting, despite adopting their numerals, number-words, and calculating methods. The Ainu, an ethnic minority in northern Japan and adjacent territories, forms its distinctive vigesimal numbers by over-counting. A fully decimal version of Ainu numbers was recorded on the island of Sakhalin, now Russian territory, and may reflect contact with Manchurian occupiers during two centuries of Manchu rule.