Fit auctions are the most structured of the auction types. The structure is determined by the agreed suit, which is fixed once a fit is agreed, even though it is possible to end up declaring in another suit at the slam level.
Most bids in a fit auction are artificial and therefore forcing.
Initially fit bidding breaks down into two zones: the game zone is the five bids up to and including the game bid, and the invitational zone is the five bids one level lower:
| Invitational zone | Game zone | |
|---|---|---|
| Hearts | 2 , 2NT, 3 , 3 , 3 | 3, 3NT, 4, 4, 4 |
| Spades | 2NT, 3, 3, 3, 3 | 3NT, 4, 4, 4, 4 |
| Clubs | 3, 3, 3, 3NT, 4 | 4, 4, 4, 4NT, 5 |
| Diamonds | 3, 3, 3NT, 4, 4 | 4, 4, 4NT, 5, 5 |