Numerical digit
From India, the thriving trade between Islamic sultans and Africa carried the concept to Cairo. Arabic mathematicians extended the system to include decimal fractions, and Muḥammad ibn Mūsā al-Ḵwārizmī wrote an important work about it in the 9th  century. The modern Arabic numerals were introduced to Europe with the translation of this work in the 12th century in Spain and Leonardo of Pisa's "Liber Abaci" of 1201. In Europe, the complete Indian system with the zero was derived from the Arabs in the 12th century.
Legacy of the Roman Empire
Roman numerals continued as the primary way of writing numbers in Europe until the 14th century, when they were largely replaced in common usage by Hindu-Arabic numerals. The Roman numeral system continues to be widely used, however, in certain formal and minor contexts, such as on clock faces, coins, in the year of construction on cornerstone inscriptions, and in generational suffixes (such as Louis XIV or William Howard Taft IV). According to the Royal Spanish Academy, in the Spanish language centuries must be written in Roman numerals, so "21st century" should be written as "Siglo XXI".