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Baleen hair is attached to the baleen plate

Baleen makes up baleen plates, which are arranged in two parallel rows that look like combs of thick hair; they are attached to the upper jaws of baleen whales. It is composed of keratin, which is the same substance that makes up human hair and nails. Whales use these combs for filter feeding. Whales are the only vertebrate group to use this method of feeding in great abundance (flamingos and crabeater seals use similar methods, but do not have baleen), and it has allowed them to grow to immense sizes. The Blue Whale, the largest animal ever to have lived, is a baleen whale.

Depending on the species of whale, a baleen plate can be 0.5 to 3.5 m (2 to 12 ft) long, and weigh up to 90 kg (200 lb). Its hairy fringes are called baleen hair or whalebone-hair. Baleen plates are broader at the gumline (base). The plates have been compared to sieves or Venetian blinds.

The word "Baleen" derives from Early Modern English word meaning "whale". It derives from the Latin balaena and is related to the Greek phallaina, both of which also mean "whale".

The oldest true fossils of baleen are only 15 million years old, but baleen rarely fossilizes, and scientists believe it originated considerably earlier than that. This is indicated by skull modifications which are associated with baleen (such as a buttress of bone found beneath the eyes in the upper jaw, and loose lower jaw bones at the chin), being found in fossils from considerably earlier. Currently, baleen is believed to have evolved around thirty million years ago, possibly from a creature with a hard, gummy upper jaw, similar to that found on Dall's porpoise today, which are, at a microscopic level, almost identical to baleen.

Curiously, many early baleen whales also had teeth, but these were likely used only peripherally, or perhaps not at all (again, similar to Dall's porpoise, which catches squid and fish by gripping them against its hard upper jaw).

A whale's baleen plates play the most important role in its filter feeding process. In order to feed, a baleen whale opens its mouth widely and scoops in dense shoals of prey (such as krill (euphausiids), copepods or small fish), together with large volumes of water. It then partly shuts its mouth and presses its tongue against its upper jaw, forcing the water to pass out sideways through the baleen, thus seiving out the prey which is then swallowed.

Baleen plates were formerly used in buggy whips and parasol ribs, and to stiffen parts of women's stays and dresses, like corsets. Baleen was commonly used to crease paper; its flexibility keeps it from damaging the paper. Its function now has been replaced by plastic.

• Baleen whale

• St. Aubin, D.J, R.H. Stinson and J.R. Geraci 1984. "Aspects of the structure and function of baleen, and some effects of exposure to petroleum hydrocarbons". Canadian Journal of Zoology 62: 193-198