Hindu-Arabic Number Notation
The system we use to write numbers today began to develop in India at the beginning of the first millennium AD and hasn't changed very much in the past 2000 years. The numbers 1 to 9 come from Hindu forms, and the Hindus also began to use a symbol to indicate nothing, which started as a l and developed into a 0 (like the present day zero). This symbol may have been inspired by the spherical counters of the abacus. A decimal system started between AD 200 and AD 600 and around AD 800 they began noting numbers down using position to indicate value and introduced negative numbers. When the Islamic Empire spread out from Arabia and encompassed North India, it is thought that Arabic mathematicians in the capital Baghdad (now the capital of Iraq) adopted the Hindu forms of number notation. As well as being great mathematicians and astronomers, the Arabs were great traders and as the Islamic Empire spread, so did their way of writing down numbers. Later Christian Europe tried to stop people adopting the Islamic method and there was even a law passed in 1299 banning traders from using Arabic numerals, but as you can see, it didn't work.