Comprehensive Rules Changes
Go, team, go! This rule defines what "your team" means, supporting some cards in the Battlebond set. It also reassures you that everything is awesome and you're still part of a team, even if you're the only player on that team.
The owner of a card is the player who started the game with that card in their deck. But what about tokens? They don't start in your deck, not unless you're doing something very wrong. Up until now, the owner of a token has been the player under whose control it entered the battlefield. We've been considering changing the rule ever since token creation was retemplated to use the "create" technology, but there was never really a good place to put it. Well, hey, core sets are a great place for small updates just like this! Now, the owner of a token is the player who created it, no matter what Crafty Cutpurse says.
112.1a and 112.10
Two rules reference gaining abilities as strictly using the words "gain" and "have." They list out all the conjugations for those verbs, but neglect that we use some templates that just don't use those words at all. Most notably, we often say that a noncreature permanent becomes a creature "with" some ability. These rules have been loosened up to better reflect reality.
This one's the rule that talks about abilities that move stuff out of non-battlefield zones and how those abilities get to work in those zones, unless they don't.
Well, that's about as clear as a brick. How about an example? This rule says that Bloodghast's triggered ability works while it's already in your graveyard and does not trigger from the battlefield. That's the first clause. Then there's a big "unless," and that's the part that says The Scarab God's ability doesn't trigger from the graveyard (it triggers from the battlefield like every other "dies" trigger).
Now that you know more about this rule than you could ever want to, we've given it a minor fixup to make sure that it works nicely with Auras that return from your graveyard when the enchanted creature dies.
117.8 and 117.8e
At some point in the past, we wanted to be very clear that some additions to costs weren't actually additional costs. Everyone on the rules team is positive that there was a very good reason for this at the time. However, the evolving rules around casting spells have rendered it vestigial, so it's time to clip it out before it gets inflamed and ruptures. If something raises a spell's cost, that's an additional cost, plain and simple.
Look at all these fresh faces! Say hello to Rowan, Vivien, Will, Yanggu, and Yanling. They're joining the list of planeswalker types.
Meanwhile, over on the list of creature types, we have Azra, Egg, and Pangolin. No word yet on whether we'll ever have an Azra Egg, Pangolin Egg, or Azra Pangolin, but you can ask Doug Beyer nicely if you'd like to see those.
No, Changelings don't count. Changelings never count.
The information about information below the text box was pretty outdated. I rewrote it to join us here in 2018.
601.2 and 601.3a
The first of these two rules handles how to begin casting a spell. It got some big changes back in the day to support Void Winnower. If the card has an even converted mana cost, but you can make the spell have an odd converted mana cost (most likely by choosing an odd value for an X), you should be able to cast it. This meant that "can't cast" effects had to be dodged somehow.
601.2 did that, but the new text wasn't quite doing exactly what we intended. That "dodge the can't" clause was a very important rule and it didn't deserve to be one clause in a wall of text, especially since the narrow space didn't let us use the right words to express what it needed to. That never really mattered until Squee got stuck on Ixalan. (Totally not canon.)
The details behind the interaction of Squee, the Immortal and Ixalan's Binding were weird, unintuitive, and not at all what the rule was meant to be. So, I unweirded it. I removed the clause about avoiding "can't cast" effects from 601.2, created a new 601.3a to handle that, and made that new rule more airtight. To get around a "can't cast" effect, you need to show that a choice made during the first few steps of casting the spell can cause the spell itself to change in such a way to no longer be prohibited.
(Remember, if you do get around the "can't cast" like this but then don't actually make that choice, 601.2e still stops you.)
You may have noticed that reflexive triggers, first appearing on Heart-Piercer Manticore, have come back in this set. This new rules technology was well received, so it's a tool designers can use all the time now. I've updated the reflexive trigger rule to include "when [something happens] this way" to cover Vivien's Invocation.
701.17c, 701.18h, and 701.22c
The "active player, nonactive player" rules way back up in 101.4 tell us all about making sequential choices when multiple players have choices to make. Otherwise, actions are simultaneous. But what does that mean for scry, search, and clash, where multiple players can do them but the steps aren't all choices? Do you scry one at a time, or take actions simultaneously until you have to make choices? These keywords gained a reminder sub-rule to clarify something we've ruled all along: if two players scry, they both look at the top card or cards, then make their choices in APNAP order.
This rule said that protection is usually from a color, but it could be from any characteristic. How thoughtful! We can also have protection from players or from everything, but those are handled specially. What's actually missing is protection from certain converted mana costs, since converted mana cost isn't a characteristic—it's a value determined by characteristics and other information. The rule has been expanded to cover pretty much anything that can be used to identify stuff. Sorry, Mistmeadow Skulk!
Everybody needs a good, good friend. Here's where we define the "partner with" variant of the partner keyword, letting the stars of Valor's Reach be happy together.
And here are where the rules for assist live, so that somebody (not just anybody) can help you cast your spells.
Zndrsplt and Okaun had some opinions on the language used to describe coin flips that you can win or lose. We didn't exactly see eye-to-eye on whether a change was necessary, but this high-profile pair in Battlebond was a good enough motivation to revise the language of these rules. Nothing's changed here functionally.
While investigating what support we'd need for Metamorphic Alteration, I realized that we never explicitly say that Clone doesn't change when the copiable values change over on the creature it's copying. You could infer it from a few other rules, but it's handy when we say things like that in one rule, and it also segues into
Metamorphic Alteration is the first time that we have a copy effect generated by a static ability rather than a one-shot spell ability, replacement effect, or such. This new rule answers the questions about what happens when the copied object leaves the battlefield—the copy effect locks in the information it's CTRL-Ving onto the other object as it starts to apply for the first time.
This rule is all about copy effects that copy an object but don't copy some detail about it. There aren't many of these left, but nothing uses "except it's still" following this set's Oracle update. This rule is still here, except its middle sentence is cut out since it describes text that doesn't exist anymore.
Several people have correctly noticed that the last ability of Tezzeret, Cruel Machinist is supported by 707.2, but fewer noticed that 707.2a goes on to contradict its parent rule like a rebellious teenager. It says that the copiable values of a face-down creature are always a 2/2 blank, no matter what! That insolent blob of text has been corrected to say that a face-down creature is a 2/2 blank unless otherwise specified.
The Magic Tournament Rules received an update with this set, like with most sets. I suspect you haven't actually seen it yet, so this is a hot exclusive preview: the MTR now contains a very slightly modified version of the loop rules, and by the way, it has contained a lot of shortcut rules for a while. This new rule here reminds you that the MTR takes precedence in tournaments. If you care about infinite loops in tournaments, keep an eye out for that document to update.
If you illegally cast a spell with assist, most likely because your assistant didn't pony up enough mana for you, you rewind the casting, put the card back where it came from, and undo the mana abilities. We've updated this rule to clarify that anyone's mana abilities are undone, not just the spell's controller's mana abilities.
Heeere's Brawl! Since its entire rule set is a child of Commander, the Comprehensive Rules consider Brawl to be an option of how to play Commander, even though in practice, it's a very different format. Then again, Standard and Legacy are very different, and the game rules call out no distinction between those two formats at all.