Blue dog 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.
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
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
- Beggar
- Armory
- Graverobber
- Messenger
- Transmogrify
- Artificer
- Seaway
