Vertical lines or ridging is a natural part of nail growth. The ridges are usually mild. But they may be more pronounced in some people’s nails. They tend to happen more as we get older.

Horizontal lines (grooves) are caused by interruptions in nail growth. This can happen from trauma or disease. The nail matrix lies underneath the visible part of the nail. The matrix produces keratin, the hard protein that makes up the nail.

Ordinarily, the nail matrix makes enough keratin. So our nails grow about 0.1 millimeter a day (or roughly three millimeters a month). This rate, like so many other things, slows with age.

If the nail matrix is damaged or its natural processes are disrupted, nail growth slows or stops. This interruption may show up as a thin, horizontal white line.

Manicures can cause horizontal lines because all the work that goes into making the nails beautiful can affect the underlying matrix. In fact, you can sometimes tell how often people get their nails done by the spacing of the lines. If they’re three millimeters apart, that means the person has a manicure every month.

Some diseases, like heart attacks, can cause indented horizontal grooves in the nail called Beau’s lines. Certain medicine can too. One example is cytotoxic drugs like those used in cancer chemotherapy.

Beau’s lines actually etch a timeline into the fingernail. By measuring the distance between the cuticle and the Beau’s lines, you can figure out how long ago the insult happened. For example, if the groove is six millimeters from the cuticle, then the health problem happened about two months ago.