The crusades of the 11th to 15th century CE have become one of the defining events of the Middle Ages in both Europe and the Middle East. The entire structure of European society changed during the 12th and 13th centuries. The Crusades were a series of religious wars between Christians and Muslims
<h2> Al-Khwārizmī became famous for his mathematical works. He wrote a book on algebra from whose title the word algebra is derived, and he wrote a book on calculation that introduced to Europe the Hindu-Arabic numerals and how to do arithmetic with them.</h2>