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 an ability has an effect either on a card it gained, or based on a card it gained, but the card was never actually gained for one reason or another. In this case, that part of the ability 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. Possession works similarly: If you would gain a card with Ironworks when you're Possessed, the Possessor gains the card instead, so you never gained anything with Ironworks, and so Ironworks gives no bonus.

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

Gain-effect cards

• Ironworks
• Horn of Plenty
• Messenger
• 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

(This explanation was later reversed, so that "it" actually does mean "the dog you fed", based on the argument that the analogy should have been "feed a blue dog", not "feed the blue dog".)