If a card has been printed at a specific rarity, it is classified under that rarity. This classification is based on paper and MTGO releases, excluding Arena.
To find out if a card is legal or not, look it up on Scryfall. If it is legal in Pauper then you can play it. If it is banned, check the Pauper Commander banlist. If it is banned there too then it is not legal, otherwise it is legal.
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