1200 Buttons invented to decorate clothing. The sport polo introduced by Persians. Fourth Crusade (till 1204). 1202 Leonardo of Pisa publishes the Book of the Abacus explaining the Hindu-Arabic system to Europeans. The system was devised by Musa al-Kwarizmi in the 8th century. (Positional Base Notation and The Zero and The Point and Negative Numbers [...]
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