It was briefly mentioned in last week’s tournament rules tip that when using double-faced cards, if you do not want to use checklists and intend to use the actual double-faced cards, you must use opaque sleeve. This bears repeating and further description, as there have been too many instances of players using sleeves that do not adequately hide the cards inside them.
Opaque means that you are completely unable to see through the backs of the sleeves and identify any portion of the card on the other side. Many colored sleeves are mostly opaque, but it is possible to see through them slightly. When not using double-faced cards, it is not imperative as long as the cards in those sleeves are not marked. In these instances, all that can be seen is the back of a Magic card.
However, when using double-faced cards, it is possible to either tell that this card is double-faced or to be able to tell what card it is if the sleeve is not completely opaque. Assuming that the use of semi-transparent sleeves is not intentional, the infraction for being able to see through these sleeves is Marked Cards. Normally the penalty for this infraction is a warning. However, this penalty can be upgraded if the head judge feels that the markings are noticeable enough that they could impact a game. Knowing that a card on top of a library is a double-faced card or a specific double faced cards is almost always grounds for applying this upgrade.
If a judge believes that a players is intentionally using sleeves that he can see through to identify double-faced cards, the infraction in this case would be Cheating – Manipulation of Game Materials, and the punishment for this would be disqualification.