Comprehensive Rules Changes
What are the Comprehensive Rules?
Magic is complicated. No, really. When you have over 12,000 interchangeable game pieces, you get some freaky interactions. The Comprehensive Rules cover everything the game has ever come up with, from basic game play structure, to every keyword ever, to entire pages dedicated to single bizarre cards (hello, Karn Liberated!) The Comprehensive Rules are, well, comprehensive ... but they're also obtuse, unfriendly, and looooong. They're not intended to be a player resource—they're a judge resource, a rules guru resource, and a place to store definitive answers. In fact, I honestly recommend never reading them. For a much friendlier rulebook that is intended to be a player resource, check out the Rules Page and download the Basic Rulebook (2.1 MB PDF). It doesn't have sections about phasing or subgames ... but you'll never miss them.
Due to an oversight, we had conflicting rules covering who controlled an emblem. 109.4 said only objects on the battlefield or the stack had a controller, with a few exceptions for command zone-based objects. Emblems are now included in those exceptions.
This rule covers triggered abilities that trigger off a player gaining life. It wasn't clear that gaining 0 life wouldn't trigger such abilities, so now the rule explicitly calls that out.
Color indicator was accidentally left off the parts of the card. Fixed!
This rule covers cases like Gutter Grime, where one object grants an ability to another object that refers to the originating object by name. In short, each Ooze knows which Gutter Grime created it. The change here is a clarification that copies of that Ooze will also look to the Gutter Grime that created the original.
We added fateful hour to the list of ability words.
We're beefing up this rule that covers Equipment legally attaching to objects. It wasn't explicit that an Equipment could equip only one creature, although there was massive evidence supporting it. Confusing the issue was the interaction of living weapon and Doubling Season, which seemed to suggest you'd create two tokens then attach the Equipment to both of them. In practice, you'd just pick one, and now the rule unambiguously tells you that.
For consistency and clarity, the changes made in 301.5 regarding Equipment were also made here regarding Auras.
This rule deals with cards that are exiled face down. It clarifies that face-down cards in exile have no characteristics (so they can't be targeted by Runic Repetition, for example) and gives a bit more detail about casting cards that have been exiled face down, namely that you turn them face up (unless you're casting them face down) just before starting the casting process.
This rule talks about certain triggered abilities having to "look back in time" to determine if they should trigger. It was streamlined a bit in this update.
This subrule is part of the layer system that handles the interaction of continuous effects. Specifically, it's part of layer 7, effects that affect power and/or toughness. More specifically, it's layer 7a, which applies effects from characteristic-defining abilities. We just expanded that to say "that define power and/or toughness" for clarity.
This rule covers the usage of "exchange" found on Tree of Redemption. It makes the Tree's toughness become equal to your former life total, but it wasn't totally clear how it did that. Now it is: by creating a continuous effect setting its toughness to that value.
This new rule handles the Dark Ascension keyword ability undying.
This new rule was added for clarification surrounding a very popular question since undying was previewed: What happens if a creature with undying and a +1/+1 counter receives enough -1/-1 counters at once (say, from Black Sun's Zenith) to cause it to die? Will undying trigger or not? The answer: No, it doesn't trigger and the creature won't return.
State-based actions all happen simultaneously, so one moment there's a creature on the battlefield with +1/+1 and -1/-1 counters, and the next moment that creature is in the graveyard. Undying asks "did that creature have a +1/+1 counter on it?" When you look back to that moment before state-based actions were performed, you see that it did, so undying doesn't trigger.
This was derivable from the rules before, but the number of questions I was getting certainly indicated that it could be clearer, so now this new rule exists.
While most things that copy a spell put that copy directly onto the stack, a few instruct you to actually cast the copy. It was never very clear where the copy existed before you cast it, so this rule now makes that clear. The copy is created in the same zone as the original object and cast from there. This information could be important to cards like Burning Vengeance.
This rule, governing how double-faced cards enter the battlefield, gets expanded to handle Loyal Cathar dying and transforming into Unhallowed Cathar.
Okay, here's a fun one. This new rule is in the section that handles a player leaving a multiplayer game. Say Mark Rosewater, Aaron Forsythe, and I are in a three-player game. I cast Bribery targeting Mark and put his Maro (what else?) onto the battlefield under my control. Aaron then casts Act of Aggression, taking control of it until end of turn. While Aaron controls it, I leave the game. Nothing happens to Maro at this point as I neither own nor control it. When Act of Aggression's effect wears off, what happens to Maro? Its original controller has left the game. It feels weird to just leave it under Aaron's control because the effect giving him control of it just wore off. It feels weird giving it to Mark because he never controlled it. I decided exiling it was the choice most consistent with other rules handling what happens to objects when a player leaves the game.
The cross-reference in this rule was pointing the wrong way.
This is the Commander mulligan rule. I inadvertently left off that you exiled the cards you don't like face down, so that bit was added.
The cross-reference in this rule didn't point you at a very informative place if you wanted to learn more, so I added some new ones that hopefully are more helpful.
Entry and definition added.