Bell-bottoms (or flares) are a style of trousers that become wider from the knees downward, forming a bell-like shape of the trouser leg.

In the early 19th century, when standardised uniforms for British ratings in the Royal Navy or enlisted men in the U.S. Navy did not as yet exist, some sailors adopted a style of wide trousers ending in bell-shaped cuffs. In 1813, one of the first recorded descriptions of sailors' uniforms, written by Commodore Stephen Decatur, noted that the men on the frigates United States and Macedonia were wearing "glazed canvas hats with stiff brims, decked with streamers of ribbon, blue jackets buttoned loosely over waistcoats, and blue trousers with bell bottoms."

The Royal Navy had often been a leader in nautical fashion, but bell-bottoms did not become part of the standard uniform until the mid-19th century. These bell-bottoms were often just very wide-legged trousers, rather than shaped trousers that flared below the knee. They continued in use as a distinctive feature of the RN rating's "square rig" uniform until replaced by more conventionally flared trousers in 1977.

Although the trousers of the present-day uniform of the United States Navy are still referred to as bell-bottomed, they simply have large straight legs. The wearer's thigh fills the upper trouser leg, making the bottom of the pants leg appear flared. This style has been popular for many years, perhaps originally because the trouser leg can be rolled up easily, allowing the wearer to work in bare feet, but there is no reliable documentation that confirms a specific timeline or reason for the popularity of bell-bottomed trousers in naval apparel.

Some modern naval uniforms continue to use bell-bottomed trousers as a potential life-saving device. The trouser material is made of cotton fibers that swell when wet and can hold air. In the event of a sailor falling overboard or having to abandon ship without a life vest, the bell-bottomed trousers can be quickly removed in the water without having to remove footwear. As part of their survival training, sailors are taught to remove the trousers while floating, tie the leg bottoms in a knot, and then use one of several methods to inflate the trousers with air. The inflated trousers can provide extra flotation while awaiting rescue.

Bell-bottoms have also been worn for centuries by European carpenters, which is explained by the fact that the widening legs prevent sawdust from falling onto their shoes or feet. Bell-bottoms are still worn by carpenters who decide to spend their time after the apprenticeship as journeymen on the road.

In the 1960s, bell-bottoms became fashionable for both men and women in London and expanded into Europe and North America. Often made of denim, they flared out from the bottom of the calf, and had slightly curved hems and a circumference of 18 inches (46 cm) at the bottom of each leg opening. They were usually worn with Cuban-heeled shoes, clogs, or Chelsea boots. Toni Basil, who was a go-go dancer when the 1964 concert film T.A.M.I. Show was released, appeared in the film wearing bell-bottoms with a baby doll blouse.

Peggy Caserta began stocking jeans from San Francisco blue-jeans icon Levi Strauss & Co. in her Haight-Ashbury store Mnasidika during the 1960s. Caserta hired a local artisan to create customized Levi's jeans with added flared inserts. Her store sold to many rising San Francisco musicians, as well as to the general hippie population. When demand for these bell-bottoms exceeded supply, she approached the nearby Levi Strauss & Co. factory, where an employee facilitated the production of flared jeans exclusively for Mnasidika. The partnership lasted through 1968, during which Caserta sold hundreds of pairs. This inspired Levi's to launch its 646 Bell Bottom jean in 1969.