A bulldog clip is a device for temporarily binding sheets of paper together. It consists of a rectangular sheet of springy steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 curved into a cylinder, with two flat steel strips inserted to form combined handles and jaws. The user presses the two handles together, causing the jaws to open against the force of the spring, then inserts a stack of papers and releases the handles. The spring forces the jaws together, gripping the papers firmly.

BULLDOG is a registered trademark

Trademark

A trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...

 of Setten IXL Limited in relationship to stationery equipment, however the term "bulldog clip" has largely become a genericised trademark for all varieties of binder clip

Binder clip

A binder clip, or a banker's clip or foldover clip, is a simple device for binding sheets of paper together. It leaves the paper intact and can be removed quickly and easily, unlike the staple...

. Its registration as a trademark in the United Kingdom dates back to 1944.