From DominionStrategy Wiki
|At the start of your turn, +.|
The Key is an Artifact from Renaissance. It gives + each turn, which is like a Treasury you can always topdeck. There is one copy of it, and players may choose to take it from each other when playing a Treasurer.
 Official FAQ
- The Key does not help you the turn you take it; it gives + at the start of your turn, which has already happened.
 Other rules clarifications
- If you play Treasurer at the start of your turn (via Prince or similar), and take the Key, you do get the + , since it's still the start of your turn.
 English versions
|At the start of your turn, +.||Renaissance||November 2018|
 Other language versions
|Chinese||金鑰 (pron. jīnyào)|
|Japanese||鍵 (pron. kagi)||あなたのターンの開始時に、+。|
|Russian||Ключ (pron. klyuch)|
 Secret History