The term Age describes the periods of civilization advancement in the Age of Empires series games. Advancement to an Age opens up new units and technologies for a civilization. Age of Empires and Age of Empires II have four Ages each, while Age of Empires III and Age of Mythology have an extra fifth. In Age of Empires III, the final Age can only be reached in a Supremacy-type game. In Age of Mythology, the fifth Age was introduced in The Titans expansion. In all three main Age of Empires games, including each expansion, the player is able to choose which Age to start from at the beginning of a game.