729.1a A “subgame” is a completely separate Magic game created by an effect. Essentially, it’s a game within a game. The “main game” is the game in which the spell or ability that created the subgame was cast or activated. The main game is temporarily discontinued while the subgame is in progress. It resumes when the subgame ends.
729.1b No effects or definitions created in either the main game or the subgame have any meaning in the other, except as defined by the effect that created the subgame. For example, the effect may say that something happens in the main game to the winner or loser of the subgame.
729.2 As the subgame starts, an entirely new set of game zones is created. Each player takes all the cards in their main-game library, moves them to their subgame library, and shuffles them. No other cards in a main-game zone are moved to their corresponding subgame zone, except as specified in rules 729.2a–c. Randomly determine which player goes first. The subgame proceeds like a normal game, following all other rules in rule 103, “Starting the Game.”
729.2a As a subgame begins, if one or more supplementary decks of nontraditional cards are being used, each player moves each of their supplementary decks from the main-game command zone to the subgame command zone and shuffles it. (Face-up nontraditional cards remain in the main-game command zone.)
729.2c As a subgame of a Commander game starts, each player moves their commander from the main-game command zone (if it’s there) to the subgame command zone.
729.3 Because each player draws seven cards when a game begins, any player with fewer than seven cards in their deck will lose the subgame when state-based actions are checked during the upkeep step of the first turn, regardless of any mulligans that player takes. (See rule 704, “State-Based Actions.”)
729.4 All objects in the main game and all cards outside the main game are considered outside the subgame (except those specifically brought into the subgame). All players not currently in the subgame are considered outside the subgame.
729.4a Some effects can bring cards into a game from outside of it. If a card is brought into a subgame from a main game, abilities in the main game that trigger on objects leaving a main-game zone will trigger, but they won’t be put onto the stack until the main game resumes.
729.4b A player’s main-game counters aren’t considered part of the subgame, although the player will still have them when the main game resumes. Similarly, any counters a player gets during a subgame will cease to exist when the subgame ends.
729.5 At the end of a subgame, each player takes all traditional cards they own that are in the subgame other than those in the subgame command zone, puts them into their main-game library, then shuffles them. This includes cards in the subgame’s exile zone and cards that represent phased-out permanents as the subgame ends. Except as specified in rules 729.5a–c, all other objects in the subgame cease to exist, as do the zones created for the subgame. The main game continues from the point at which it was discontinued: First, the spell or ability that created the subgame finishes resolving, even if it was created by a spell card that’s no longer on the stack. Then, if any main-game abilities triggered while the subgame was in progress due to cards being removed from the main game, those abilities are put onto the stack.
729.5a At the end of a subgame, each nontraditional card not in a supplementary deck that began the subgame in a supplementary deck is turned face down and put on the bottom of that deck. Then each player moves each of their supplementary decks from the subgame command zone to the main-game command zone and shuffles it.
729.5b At the end of a subgame of a Vanguard game, each player moves their vanguard card from the subgame command zone to the main-game command zone. This is an exception to rule 313.2.
729.5c At the end of a subgame of a Commander game, each player moves their commander from the subgame command zone (if it’s there) to the main-game command zone.