Nearly every Greek city had its own games, but not all enjoyed the same prestige. In the course of the history of the games, some contests grew to be more important than others. From the sixth century onwards, four great international games distinguished themselves because of their more international catchment area, and because of the prestige attached to victories obtained here. The Olympic and Pythian games were held every four years, the Nemean and Isthmian games were biennial. In these games only crowns of leaves were given as a symbolic prize - for this reason fourth-century writers of literature describe them as 'crown games' - but a victor in one of these games also received important rewards in his home town. These four games formed a kind of circuit: the Isthmian games took place in the spring of the same year as the Olympic and Pythian games and were therefore a kind of preparation for the more prominent quadrennial games; the Nemean games took place in the alternative years.
In the third century BC, there was a upsurge of new contests. The political landscape had changed drastically since the conquests of Alexander the Great, and many cities, mostly towns in Greece and what today is Turkey, but even Alexandria in Egypt, wanted to place themselves in the limelight by introducing their own international contests. They negotiated with other cities to ensure that a victor of their new games would receive in his home town the same rewards as for a victory in the Olympic or Pythian games. In this period, 'crown games' became a term of the technical athletic jargon denoting these new is-Olympic and iso-Pythian games. The traditional top four remained more prestigious, however, and in the second century BC a technical term was developed for this group as well, namely periodos (i.e. the circle or circuit). Being a periodos-winner was an achievement comparable to winning a Grand Slam in modern tennis.
In the Roman period this 'old periodos' was extended by new quadrennial games: the Actia near the town Actium, the Sebasta at Naples, the Capitolia in Rome and the Eusebeia at Puteoli. A 'full periodos' in the Roman period therefore includes eight international games. The new games were all celebrated in year 2. The Nemean games of the second year therefore had to be moved to december. Hence Pausanias calls them 'winter-Nemea'.
The games of the periodos formed the basis of a fixed four-year calendar in which all the other games were inserted. A logical sequence of local and international games made it possible for the athletes to travel from one contest to the other. Recent discoveries help us to understand how the Greeks fixed the dates of the games: in a shipwreck archaeologists have discovered a bronze mechanism with gears, a complex scientific instrument for calendrical purposes, such as determining the date of the Olympics.