Persian is a very old language, and linguists use names for three different versions of Persian that were used in three different times. Old Persian was spoken in the first Persian Empire, under the Achaemenid kings, including Cyrus and Darius the Great. The empire existed from the 6th century BC to the conquest of Alexander the Great. The second Persian empire was ruled by the Sassanian kings from the 2nd century AD until the Muslim conquest of Iran by the Arabs in the 7th century and spoke Middle Persian, or Pahlavi.