721.1. One keyword causes an object to merge with a permanent. See rule 702.140, “Mutate.”
721.2. To merge an object with a permanent, place that object on top of or under that permanent. That permanent becomes a merged permanent represented by the card or copy that represented that object in addition to any other components that were representing it.
721.2a A merged permanent has only the characteristics of its topmost component, unless otherwise specified by the effect that caused them to merge. This is a copiable effect whose timestamp is the time the objects merged. (See rule 613.2.)
721.2b As an object merges with a permanent, that object leaves its previous zone and becomes part of an object on the battlefield, but the resulting permanent isn’t considered to have just entered the battlefield.
721.2c Because a merged permanent is the same object that it was before, it hasn’t just come under a player’s control, any continuous effects that affected it continue to do so, and so on.
721.2d If a merged permanent contains a token, the resulting permanent is a token only if the topmost component is a token.
721.2e If a merged permanent contains face-up and face-down components, the permanent’s status is determined by its topmost component. If a face-down permanent becomes a face-up permanent as a result of an object merging with it, other effects don’t count it as being turned face up.
721.2f If a merged permanent is turned face down, each face-up component that represents it is turned face down. If a face-down merged permanent is turned face up, each face-down component that represents it is turned face up.
721.2g A face-down merged permanent that contains an instant or sorcery card can’t be turned face up. If such a permanent would turn face up, its controller reveals it and leaves it face down. Abilities that trigger when a permanent is turned face up won’t trigger.
721.2h If a merged permanent contains a flip card (see rule 709), that component’s alternative characteristics are used instead of its normal characteristics if the merged permanent is flipped.
721.2i If a merged permanent contains one or more transforming double-faced cards (see rule 711), transforming that permanent causes each of those double-faced cards to turn so that its other face is up.
721.2j A face-up merged permanent that contains a double-faced card or meld card can’t be turned face down.
721.3. If a merged permanent leaves the battlefield, one permanent leaves the battlefield and each of the individual components are put into the appropriate zone.
721.3a If a merged permanent is put into its owner’s graveyard or library, that player may arrange the new objects in any order. If it’s put into its owner’s library, that player doesn’t reveal the order.
721.3b If a player exiles a merged permanent, that player determines the relative timestamp order of the cards at that time. This is an exception to the procedure described in rule 613.7k.
721.3c If an effect can find the new object that a merged permanent becomes as it leaves the battlefield, it finds all of those objects. (See rule 400.7.) If that effect causes actions to be taken upon those objects, the same actions are taken upon each of them.
721.3d If multiple replacement effects could be applied to the event of a merged permanent leaving the battlefield or being put into the new zone, applying one of those replacement effects to the object applies it to all components of the object. If the merged permanent is a commander, it may be exempt from this rule; see rule 903.9a.
721.3e If a replacement effect applies to a “card” being put into a zone without also including tokens, that effect applies to all components of the merged permanent if it’s not a token, including components that are tokens. If the merged permanent is a token but some of its components are cards, the merged permanent and its token components are put into the appropriate zone, and the components that are cards are moved by the replacement effect.