A marked card is a card that can be identified without seeing the front of the card. This includes but is not limited to warping, creases, discoloration, card thickness or texture, and water-marks. If the cards are sleeved, this includes but is not limited to sleeves with identifying marks or other unique characteristics that distinguish it from other cards in the Deck. Cards that have been physically altered to add or subtract layers of foil, etc, are not legal for sanctioned tournament play.
If players are using sleeves to protect their cards, they should take extra precautions to ensure that their sleeves do not become marked during the course of a tournament. The head judge may require a player to re-sleeve their Deck should they feel that the sleeves are marked. The head judge may require a player to replace a marked card(s) during the course of a tournament. If the player cannot replace the card(s) before their next Match begins, appropriate tardiness penalties at three minutes and ten minutes will begin to apply, as the player no longer has a Deck that matches their registered Deck list. The tardiness penalties are in addition to the penalties that were applied for marked cards.
On extremely rare occasions, a player may pull a miscut or misprinted card from a product. These cards are not considered to be tournament legal if they can be identified without seeing the front of the card. If they cannot replace the card(s), or choose not to replace the card by the 10-minute mark in the Match, they will be marked as a no-show and dropped from the tournament. If the player does not wish to drop from the tournament, they must notify the scorekeeper.
Mai Valentine scented her cards to know which ones she was going to draw soon. In the English dub, Seeker applied an invisible ink to his cards to allow him to see what cards he would draw next (in the original Japanese version, these cards were counterfeit). Arkana trimmed the edges of his cards so that "Dark Magician" always came to the top.
In Yu-Gi-Oh! 5D's - Episode 116 another example of marked cards was shown during Yusei's and Clark Smith's Concentration Duel when Clark marked his cards so that their edges glowed in different colors depending on the card type when seen with his glasses. Clark used this to cheat against Yusei, but Yusei realized it and used Clark's trick against him to win.