Duel Commander follows the exact same rules as classic multiplayer Commander, except for the following.
First, players start the game with 20 life.
Second, in addition to the ability to put the Commander back into the Command Zone if it would go to the graveyard or be exiled, you may do so also if it would be put into your library.
Third, new cards are legal on release day (Friday following the pre-release).
Finally, there is a specific banlist.
- Matches are best-of-three.
- Since games can take more time than smaller formats, we recommend a time limit of at least 55 minutes.
- No sideboards are used.
Apart from the points mentioned above, all the rules of Duel Commander obey the rules of classic multiplayer Commander. This includes poison counters rule (10 counters needed to kill a player).
Why specific rules for Duel Commander ?
When you play a game with more than two players, social interactions, diplomacy and negotiations usually prevent a player from taking a huge advantage at the start of the game. Indeed, if a players becomes the main threat at the table, it is likely that he will become the top priority target for other players. Such a player should have to face several opponents at the same time, which brings balance in the game. Multiplayer games tend to be balanced by diplomacy.
When only two players are facing each other, this cannot be done. If a player takes a great advantage early in the game (example : turn one Island, Lotus Petal, Mana Crypt, Jace the Mind Sculptor), his opponent won’t be able to get back in the game. That is why we banned certain cards which lead to extraordinary early game advantages from which an opponent cannot recover (Mana Crypt, Sol Ring…). Some cards are banned as commanders for this purpose : those cards can still be played in your deck, but can’t be chosen as a commander. We also removed certain cards which can easily lead to degenerated combos, such as Hermit Druid.
Sometimes a tournament is organized for Duel Commander. Such events need to put a time limit in rounds, generally 55 or 60 minutes per round. Considering the time restrictions, we banned Sensei’s Divining Top and Shahrazad, because they tend to make games last considerably longer, leading to numerous unfinished games at the end of the round.
You can find more complete explanations in the Duel Commander Rules Summary.
Reasoning behind the banlist can be found in this article.