Originally and alternatively known as Elder Dragon Highlander, or EDH, Commander is a multiplayer Constructed format of the Magic: the Gathering trading card game. It is characterized by the use of a Commander, or General, and the 100-card singleton deck.

Deck Construction

There are four rules for building a commander deck:

Commander/General: The deck must include a Commander, also known as a general, which must be a Legendary Creature. "Commanderness" is an inherent quality of the Commander: it can neither be removed nor copied, and a Commander is still a Commander even if that Commander is being controlled by another player.
Color Identity: The deck cannot include any cards that feature mana symbols not found on the Commander, e.g. a deck whose Commander is Alesha, Who Smiles at Death may include cards that contain Mountain and/or Swamp and/or Plains symbol but may not contain cards that include Forest and/or Island symbols; the exception to this rule is reminder text, e.g. a deck whose commander is Gonti, Lord of Luxury may contain a Crypt Ghast, even though the reminder text includes a Plains symbol.
Card Copies: Except for basic lands and cards that mention otherwise, such as Relentless Rats or Shadowborn Apostle, a deck may not contain more than one copy of any card.
Size: A Commander deck must be exactly 100 cards, including the Commander. In other words, an EDH deck consists of one Commander (or two if both have Partner) and 99 (or 98) other cards.

There is also an official banlist for Commander, and a deck should not include any cards on the banlist without prior consent of other players of a particular instance of a Commander game.

Gameplay

The game starts as other Magic formats with players shuffling their decks, cutting the decks of their opponents, and drawing an initial hand of seven cards. Given that Commander is traditionally a multiplayer format, all players draw a card during their first turn. The only differences between EDH gameplay and traditional gameplay concern the Commander:

The Commander begins the game in the Command Zone, and may be cast at any legal time that its controller has sufficient mana. If at any point the Commander would be removed from the battlefield and placed into a different zone (e.g. graveyard, hand, library, or exile), its controller may instead redirect the Commander to the Command Zone. When casting a Commander from the Command Zone, a Commander's mana cost is equal to its printed mana cost plus two colorless mana for each time it has been previously cast from the Command Zone; this is colloquially known as the "command tax". However, when casting a Commander from anywhere other than the Command Zone, its cost is equal to its printed cost. Finally, if any player takes 21 or more combat damage from a single Commander over the course of a game, that player loses in spite of that player's current life total.