Magic The Gathering: Commander (2015)- Rules of Play says:
Choose your commander for this unique multiplayer Magic format. New cards offer new play options, and experience counters will make your commander grow in power during the course of each game. Two-color decks allow for surprising synergies and opportunities to devastate your foes.
The set features five two-color—enemy pair—100-card Commander decks, along with 55 new Magic cards legal in Eternal formats (Vintage and Legacy), but not legal in Standard, Block Constructed, or Modern. Each deck will contain fifteen new cards.