Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections—the upper and lower mandibles—are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes known as nares lead to the respiratory system.
Although the word beak was, in the past, generally restricted to the sharpened bills of birds of prey, in modern ornithology, the terms beak and bill are generally considered to be synonymous. The word, which dates from the 13th century, comes from the Middle Englishbec, which itself comes from the Latin beccus.