Players play with the top card of their libraries revealed.
Legalities
Legal
Duel
Predh
Legacy
Vintage
Commander
Oldschool
Oathbreaker
Not Legal
Brawl
Penny
Future
Modern
Pauper
Alchemy
Pioneer
Explorer
Historic
Standard
Timeless
Gladiator
Premodern
Standardbrawl
Paupercommander
Rulings
2013-04-15
If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, the new top card won't be revealed until you finish doing so.