Partner

Partner is a keyword ability specifically used in the Commander format. Partner was introduced in Commander 2016.

Description
A player can have two commanders if both have partner. Because both commanders start the game in the command zone, the remaining library is only 98 cards. Your two commanders are treated individually for all other Commander rules. If either leaves the battlefield, you can return it to the command zone instead. The "commander tax", the additional cost of 2 for every time you've cast a commander from the command zone, applies individually. Combat damage dealt by each commander is similarly tracked individually. If either commander can deal 21 or more combat damage to a single player over the course of the game, that player loses.

If any card refers to your commander (or commanders in general), it means either one of the two.

Partner was heavily featured in Commander Legends, where it appeared on 40 monocolored (and one colorless) legendary creatures and planeswalkers.

Legendary partner
The non-legal Heroes of the Realm card Sol, Advocate Eternal experimented with legendary partner (You can have two commanders if [this] is one of them. The other one is promoted to legendary.)

Rules
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Partner with
Battlebond introduced this variant for Two-Headed Giant. It limits which other card can be used as co-commander, but allows a player (expected to be on your team) to search their library for the [other card] and put it in their hand. Cards with "partner with" always appear alongside their counterpart in booster packs. Note that not all the partners in Battlebond are legendary. The ones that aren't can't be commanders.

Another cycle with this variant appeared in Commander 2020.