Historical Rules · 10 June 2022
Rule 712.3
Official source text from the 10 June 2022 Rules release
Players must ensure that double-faced cards in hidden zones are indistinguishable from other cards in the same zone. To do this, the owner of a double-faced card may use completely opaque card sleeves and/or a substitute card (see rule 714). Sanctioned tournaments have additional rules for playing with double-faced cards. See rule 100.6.