Planeswalker Deck “extra cards” are legal in Standard.

The Kaladesh set, in addition to the usual new cards and mechanics, also introduced a new addition to Wizards' preconstructed product line: the Planeswalker Deck, which replaces the Intro Pack as the preconstructed product released with every set (which themselves replaced Theme Decks, for those of you who remember that far back). These Planeswalker Decks don't just show off the star characters of the set, however; for the first time in Magic's history, they contain cards that are not reprints,

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