Page "Contract bridge" Paragraph 10

Bidding is based on the premise that the lowest contract available to bidders starts with the proposition to take seven tricks, i. e. one cannot contract to make less than seven tricks.

Given this, the bidding is said to start at the one-level when contracting for a total of seven tricks, at the two-level for eight tricks and so on to the seven-level to contract to take all thirteen tricks.

The six tricks required as the base for any bid are referred to as the “ book ”.