Blue dog rule

Trader, the card that most often invokes this rule.

The "blue dog rule" is a ruling by Donald X on what to do when a card has an effect either on a card it's gaining, or based on a card it's gaining, but the card to-be-gained is never actually gained for one reason or another. In this case, the first card has no effect, since it did not gain a card.

The archetypal example of this rule is the interaction between Ironworks and Trader. If you would gain a card with Ironworks, and then reveal Trader to gain a Silver instead, you never actually gained anything with Ironworks, so Ironworks fails to give a bonus effect based on the type of the card gained.

Many cards gain a card to the top of your deck or to your hand; if you reveal Trader, or are being Possessed, the Silver (or the gained card) goes into your discard pile (or the player to your right's discard pile).

It is called the "blue dog" rule because when Hinterlands was first released, Donald X posted a bizarre anecdote about walking a blue dog to try to explain the interaction.

List of cards that can invoke the Blue dog rule

Gaining-hinderers

• Possession
• Trader

If a chosen Supply pile is empty, this also hinders gaining.

Gain-effect cards

• Bureaucrat
• Mine
• Ironworks
• Trading Post
• Treasure Map
• Explorer
• Bag of Gold
• Tournament
• Develop
• Beggar
• Armory
• Graverobber
• Taxman
• Messenger
• Transmogrify
• Artificer
• Seaway
• Summon

Trivia

Donald X.'s original quote:

If I say, "feed the blue dog, then take it for a walk," "it" refers to the blue dog, including its blueness, but "it" does not mean "the dog you fed." We are talking about a blue dog, and I am giving you two commands concerning it. Feed it, walk it.

—Donald X., Ironworks and Trader