Donatello, the Brains

Partner abilities are keyword abilities that modify the rules for deck construction in the Commander variant (see rule 903), and they function before the game begins. Each partner ability allows you to designate two legendary cards as your commander rather than one. Each partner ability has its own requirements for those two commanders. The partner abilities are: partner, partner with [name], friends forever, choose a Background, and Doctor’s companion.

Your deck must contain exactly 100 cards, including its two commanders. Both commanders begin the game in the command zone.

A rule or effect that refers to your commander’s color identity refers to the combined color identities of your two commanders. See rule 903.4.

Except for determining the color identity of your commander, the two commanders function independently. When casting a commander with partner, ignore how many times your other commander has been cast (see rule 903.8). When determining whether a player has been dealt 21 or more combat damage by the same commander, consider damage from each of your two commanders separately (see rule 903.10a).

If an effect refers to your commander while you have two commanders, it refers to either one. If an effect causes you to perform an action on your commander and it could affect both, you choose which it refers to at the time the effect is applied.

Different partner abilities are distinct from one another and cannot be combined. For example, you cannot designate two cards as your commander if one of them has “partner” and the other has “partner with [name].”

If a legendary card has more than one partner ability, you may choose which one to use when designating your commander, but you can’t use both. Notably, no partner ability or combination of partner abilities can ever let a player have more than two commanders.

“Partner” means “You may designate two legendary creature cards as your commander rather than one if each of them has partner.”

“Partner with [name]” represents two abilities. It means “You may designate two legendary creature cards as your commander rather than one if each has a ‘partner with [name]’ ability with the other’s name” and “When this permanent enters, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle.”

“Friends forever” means “You may designate two legendary creature cards as your commander rather than one if each of them has friends forever.”

“Choose a Background” means “You may designate two cards as your commander rather than one if one of them is this card and the other is a legendary Background enchantment card.” You can’t designate two cards as your commander if one has a “choose a Background” ability and the other is not a legendary Background enchantment card, and legendary Background enchantment cards can’t be your commander unless you have also designated a commander with “choose a Background.”

“Doctor’s companion” means “You may designate two legendary creature cards as your commander rather than one if one of them is this card and the other is a legendary Time Lord Doctor creature card that has no other creature types.”

If an effect refers to a partner ability by name, it means only that partner ability and not any others. If an effect refers to the partner ability or cards with partner and doesn’t mention a specific variant of the partner ability by name, it is referring only to partner, partner with [name], or cards with either of those abilities, and does not refer to any other partner variant.