Beau’s lines. Beau’s lines are deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate.

This is a disorder caused by an acute illness. As a result, the matrix temporarily stops producing new cells for the duration of the illness. When it once again begins to reproduce, the period of the illness is clearly marked by a definite furrow or series of furrows.

This grows forward and eventually disappears as it is cut away as part of the free edge.