Playtest table
What face-down cards and the library wear on the table, and what lies under the battlefield. Card backs come from the MPC Autofill community; a mat can be any card’s art, or a picture you link.
Contributing
Your finished decks help improve suggestions.
Suggestions today come from a popularity list: how often Commander decks play each card. That knows Sol Ring is in everything. It does not know which cards people put in decks like yours.
A pile of real decklists is what answers that. Once enough have been collected, “played a lot” can become “played alongside your commander, by people who built the same deck you are building” — and Modern and Standard can be suggested at all, which popularity cannot do because nobody publishes a ranking for them.
What gets stored is one line per card, plus the format. Not the deck's name, not when you built it, nothing about you. Only decks that are complete and legal, because half-built ones would teach it that the average deck is thirty cards with no lands.