Did you know that Indo-Arabic numerals are the most widespread numbering system in the world? It was invented by Indian mathematicians and introduced to the Western world in the 12th century. The system is based on 10 symbols. This free language course also covers the Arabic letters Seen ( س ) and Sheen ( ش ), the 12th and 13th letters in the Arabic alphabet. Learn how to differentiate between these two letters and use them in sentences.