Historical Rules · 1 June 2014
Rule 711.9
Official source text from the 1 June 2014 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 or substitute a checklist card. Sanctioned tournaments have additional rules for playing with double-faced cards. See rule 100.6.