Last updated July 17th, 2018
Cavern of Souls
If a player uses Cavern of Souls to cast a creature of the chosen type, it’s assumed they use the “can’t be countered” ability unless they explicitly state otherwise.
The only exception is if they require the colorless mana to cast the spell.
Identical Permanents with Non-Visual Differences
When two identical permanents are on the battlefield and there are non-visible differences between them, if a player needs to know which is which, they need to ask for clarification.
If a player has an Inkmoth Nexus, then plays a second Inkmoth Nexus, then animates one of them and attacks with it, it can be assumed that it is the one without summoning sickness. If at any point this is not clear to the opponent, they need to ask for clarification which Nexus is activated.
Self-Mill Triggers
If a trigger puts a number of cards from your library into your graveyard and the card itself has no other abilities that reward this action, the trigger is considered detrimental.
To determine whether a trigger is detrimental or not, we only look at the context of the card itself. We don’t look at the current game state or the strategy of the deck. Self-mill triggers theoretically bring you closer to being decked without any benefit on their own. So Sidisi, Brood Tyrant‘s self-mill trigger is not detrimental because the other ability on the card rewards you for milling. But Stitcher’s Supplier‘s self-mill trigger is detrimental because the card by itself does not reward you for milling yourself, even though the card would see much less play without the ability. “Is this card better or worse without this trigger?” or “Is this card played because of or in spite of this trigger?” are useful rules of thumb but they don’t work nicely with self-mill triggers, as you only take into account the context of the card itself, not the context of the deck.
Contextually detrimental triggers
If a trigger would contextually be both detrimental and beneficial, it is considered beneficial.
An example is Duskwatch Recruiter/Krallenhorde Howler. Transforming from night side to day side is usually considered detrimental on werewolves as most werewolves have a night side that is strictly better than the day side. But for this card transforming from day side to night side, or from night side to day side, means gaining one beneficial ability but losing another beneficial ability (the digging for creatures on the day side and the cost reduction on the night side). Which of these abilities is better is highly dependent on the state of the game, context we don’t take into account, so neither of these triggers are considered detrimental. The classic example is Dark Confidant. It doesn’t matter whether the controller is at 20 or 2 life. Simply asking “Would Dark Confidant be a better or worse card without this triggered ability?” shows that it’s a beneficial trigger.
Loops that require a specific number of iterations
If a player can demonstrate that a specific game state can be reached after a certain number of iterations, they do not have to provide that exact number but can shortcut to the desired game state.
For example with an arbitrarily large amount of mana, a player can activate Duskwatch Recruiter an arbitrarily large amount of times. They could then proceed to stack their entire library (as long as the number of cards in their library is not a multiple of 3), but this requires a specific number of iterations and not more. For example if 2537 iterations would put the library in the order the player wants, 2538 would be too much because then the top 3 cards would be put on the bottom again. Even though the MTR section on Loops states “If one player is involved in maintaining the loop, they choose a number of iterations.” the player is not required to provide the exact number 2537. What matters is that the loop is deterministic (the number 2537 can be calculated) so they only have to demonstrate the principle and can shortcut to the desired game state. The same is true for looping Petals of Insight.
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