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French physician Joseph Honoré Simon Beau first described "Beau's lines," or transverse ridges -- horizontal grooves on the nail -- in 1846. Beau was best known for his research on the heart and lungs, but the nail community remembers him for noticing this condition.
Beau’s lines, as they have come to be known, occur when the nail matrix is injured and the growth of the nail is slowed. This eventually causes grooves in the surface of the nail plate. Affecting finger- and toenails, this condition is most commonly caused by an injury such as slamming a finger in the door or dropping a heavy object on a toe.
Transverse ridges can also be a sign of a systemic illness—an illness that affects the entire body rather than just one organ—or of a congenital disease, which means it was acquired during the body’s development in the uterus.
Whatever the reason for the slowing or halting of nail growth, it will result in transverse ridges on the nails, which look like pale bands lying horizontally across the nail plate. They first appear at the moon (lunula) and then progress with the growth of the nail. In cases where the ridges are not caused by a systemic illness or congenital disease, Beau’s lines will grow out with the nail and disappear.
Fingernails take approximately nine months—about one millimeter per week—to grow out completely, with the middle fingernail growing most rapidly, and toenails take approximately 18 months to grow out. With this in mind, by measuring the distance from the transverse groove to the cuticle, you can sometimes estimate the date of the injury or illness.