Bascule bridge
A bascule bridge is a moveable bridge
Moveable bridge
A moveable bridge is a bridge that moves to allow passage for boats or barges. An advantage of making bridges movable include lower price, due to the absence of high piers and long approaches. The principal disadvantage is that the traffic on the bridge must be halted when it is opened for passages...

 with a counterweight that continuously balances the span
Span (architecture)
Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge.A span can be closed by a solid beam or of a rope...

, or "leaf," throughout the entire upward swing in providing clearance for boat traffic.

Bascule is a French
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 term for seesaw
A seesaw is a long, narrow board pivoted in the middle so that, as one end goes up, the other goes down.-Mechanics:Mechanically a seesaw is a lever and fulcrum....

 and balance
Weighing scale
A weighing scale is a measuring instrument for determining the weight or mass of an object. A spring scale measures weight by the distance a spring deflects under its load...

, and bascule bridges operate along the same principle. They are the most common type of movable bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

 in existence because they open quickly and require relatively little energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 to operate.

Bascule bridges may be single or double leaf.