The definition of Mendicant legality is simple. To be Mendicant-legal, a card must:

  • Exist on Magic Online, and
  • Exist on Magic Online as a common in a core set or rotational expansion set.

However, this simplicity belies the proliferation of set types that Magic: the Gathering has witnessed in recent years. As of this writing, Scryfall's API documentation lists 22 distinct set types. Most notably, cards that exist as commons on MTGO only in the following set types are not Mendicant-legal:

  • Commander
  • Draft Innovation
  • Masters
  • Treasure Chest

If in doubt, consult the legality checker provided on this website's main page.