Historical Rules · 14 April 2023
Rule 702.124h
Official source text from the 14 April 2023 Rules release
“Choose a Background” is a variant of the partner ability that represents a static ability. “Choose a Background” means “You may designate two cards as your commander as long as one of them is this card and the other is a legendary Background enchantment.” You can’t designate two cards as your commander if one has a “choose a Background” ability and the other is not a Background enchantment, and legendary Background enchantments cannot be your commander unless you have also designated a commander with “choose a Background.”