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There are several questions on BCG Stack Exchange that ask about information which was previously common but "vanishes", even though it can be deduced from other common information.

For example,

The answers generally indicate that the opponent must be able to deduce the card identity from what they knew before combined with what they know now ("derived information") and one of the questions involves a situation with coloured markers that happen to coincide with the card's colour identity.

Is there anything that disallows or allows or even requires markers to be used to indicate which of a group of facedown cards previously had a particular identity? Even further, is the player allowed to place markers on cards under the opponent's control to assist in tracking these previously known identities? An answer for any rules enforcement level is acceptable. This is most likely to be an argument at Regular, but would also have importance at Competitive and Professional.

For example, another question asked about whether the fact that a particular card was the commander remained public even if the card is facedown. Such a card could be marked with a C token to remind players of that fact. Other situations involve revealing a card that the opponent would want to ensure they can track in case they obtain a means of removing it later, and want to place a target-shaped marker on it.

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Is there anything that disallows or allows or even requires markers to be used to indicate which of a group of facedown cards previously had a particular identity?

A player must ensure at all times that their face-down spells and permanents can be easily differentiated from each other, but a physical marker is not necessarily required to do so. If just one card needs to be tracked, its controller could say "I'm going to keep the one you've seen on the side by itself."

707.6. If you control multiple face-down spells or face-down permanents, you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other. This includes, but is not limited to, knowing what ability or rules caused the permanents to be face down, the order spells were cast, the order that face-down permanents entered the battlefield, which creature(s) attacked last turn, and any other differences between face-down spells or permanents. Common methods for distinguishing between face-down objects include using counters or dice to mark the different objects, or clearly placing those objects in order on the table.

(Emphasis mine.)

Even further, is the player allowed to place markers on cards under the opponent's control to assist in tracking these previously known identities?

I can't find anything that would allow this serious breach of gaming etiquette in the rules (Comprehensive or Tournament). People can be very picky about their physical area of control, and that should be respected.

If the identification is truly unclear, you may request that the identification be made clearer (bringing in a judge if necessary), but you can't mandate the use of a specific scheme.

If you are having problems keeping track of everything, keep in mind that you may take notes.

2.11 Taking Notes

Players are allowed to take written notes during a match and may refer to those notes while that match is in progress.

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You can, and arguably must mark your face down cards

From the Comprehensive Rules

707.6. If you control multiple face-down spells or face-down permanents, you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other. This includes, but is not limited to, knowing what ability or rules caused the permanents to be face down, the order spells were cast, the order that face-down permanents entered the battlefield, which creature(s) attacked last turn, and any other differences between face-down spells or permanents. Common methods for distinguishing between face-down objects include using counters or dice to mark the different objects, or clearly placing those objects in order on the table.

emphasis mine

As you can see here, you must clearly distinguish between unique face-down cards in such a way that all players have full access to the known information about them. Physical markers are explicitly allowed as an option.

It is the responsibility of the player to ensure that their cards are sufficiently marked. It would not really be in an opponent's purview to place markers on the facedown cards themselves, but an opponent could declare that the player had not sufficiently marked the cards and demand a more descriptive marking system.

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  • Yes, but what about "is the player allowed to place markers on cards under the opponent's control to assist in tracking these previously known identities?"
    – ikegami
    Sep 15, 2019 at 4:26
  • It's open for interpretation, but since a player is responsible for their own marking, I would say that adding markers to your opponent's cards is either disallowed entirely or extremely poor edicit, but you could require that your opponent use a specific marking scheme which amounts to the same thing.
    – Arcanist Lupus
    Sep 15, 2019 at 4:45
  • Re "you could require that your opponent use a specific marking scheme", Oh yeah? What rule is that? That would definitely make the answer yes.
    – ikegami
    Sep 15, 2019 at 4:54
  • @ikegami, well your opponent needs to make all the known information about the cards publically accessible and all the objects clearly distinguished. So if you declared that their existing marking system was insuffient to meet that criteria, they'd have to replace it with a better one. Technically, you couldn't require a specific scheme, but since the markers are for the opponent's benefit I would expect most judges to side with an opponent who declared themself unsatisfied unless they're being obviously difficult and obstructive.
    – Arcanist Lupus
    Sep 15, 2019 at 5:12
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    In other words, if I wanted you to use red, green, and blue stones to mark the cards and you used red, purple, and orange meeples instead there would be nothing I could do (unless I was colorblind and found purple and red indistinguishable). But if I knew that one morph creature was actually a Willbender I could request that the Willbender gets a special marker and a judge would probably side with me.
    – Arcanist Lupus
    Sep 15, 2019 at 5:18

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