Beau's Lines:
Information for adults

Beau's lines occur along the fingernails and the toenails. The grooves extending across the nail plate often span its entire breadth. The involvement of multiple nails may suggest a systemic cause, including a side effect from medication, but the phenomenon may be limited to just the thumb nails and big toe nails. 

It is possible to measure the distance from the area where the cuticle is seen and where the nail originates (the proximal nail fold) to the leading edge of the Beau's line to determine how much time has elapsed since the line was formed. Because fingernails grow at a rate of approximately 0.1 mm per day, and toenails grow 0.03 mm per day, the duration of the causative insult can be inferred from the width of the Beau's furrow itself.

Recurrent disease will produce repeated transverse grooves, separated by normal nail.