Signature cards are the cards with the highest synergy ratings (see next question). Synergy ratings are based on how popular a card is with a given commander vs. how popular that card is with decks in general. For instance, Leyline of Singularity has a very high synery rating with Empress Galina because it is run in very many Empress Galina decks, but very few blue decks.
Synergy is calculated by taking the percentage of the times that card is in this pool of decks, subtracted by the percentage of times that card is in other decks of the same color identities. For example, If Basilisk Collar has a synergy score of +27% synergy score in Olivia Voldaren, it's because 41% of Olivia Voldaren decks play the card while 14% of Black/Red decks play it (41% - 14% = 27%). The higher the synergy score, the more this card is "special" to this commander, which is different than a top card like Sol Ring.
Whenever a deck is submitted from TappedOut or Deckstats, it is recorded in our database.
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