In the middle 800s, Cyril and Methodius became missionaries to the Slavic peoples, aiming to convert them from paganism to Christianity. They were Greek and came from the highly literate Byzantine culture (named for the city of Byzantium, also known as Constantinople, now Istanbul). To be effective the missionaries had to learn local languages, and to devise an alphabet so the languages could be written down. Cyril came up with an alphabet for Russian, based on the Greek alphabet. The alphabet is called Cyrillic, after St. Cyril. It was further developed by later missionaries, and eventually assumed its current form, with a number of letters added to represent sounds in Russian not occurring in Greek. Several letters were supposedly designed by tsar Peter the Great. The alphabet is used by more than 250 million people, and is the script used for Russian, Belarusian, Ukrainian, Bulgarian and for other languages. Cyrillic and the Orthodox church are the distinctive marks of many Slav nations.