By the way, doctor: What do these lines on my fingernails mean?
Q. Would you please tell me what it means when fingernails have vertical or horizontal lines?
A. Vertical lines or ridging is a natural part of nail growth. These lines may be more pronounced in some people's nails, particularly as they get older, but the ridges are usually mild. More severe, deep lines and splits may be seen with certain skin diseases, such as lichen planus.
Horizontal lines, or transverse grooves, are caused by interruptions in nail growth, either from trauma or disease. The nail matrix, which lies underneath the visible part of the nail, produces keratin, the hard protein that makes up the visible part. Ordinarily, the nail matrix makes enough keratin so our nails grow about 0.1 millimeter a day (or roughly three millimeters a month), although this rate, like so many things, slows with age.