By the way, doctor: What do these lines on my fingernails mean?
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.
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