A line on your nail is usually harmless, but the type of line matters. Vertical ridges running from base to tip are the most common and are a normal part of aging. Horizontal lines, dark streaks, and color changes can sometimes signal a health issue worth investigating. The key is knowing what each type looks like and what it may indicate.
Vertical Ridges: Usually Normal
Vertical ridges run lengthwise from the base of your nail to the tip. They’re by far the most common type of nail line, and most people develop them naturally as they age. Think of them like wrinkles for your nails. The nail matrix (the tissue under your cuticle that produces the nail plate) gradually becomes less uniform over time, creating these fine raised lines.
That said, vertical ridges can occasionally point to something else. Very dry skin or eczema can cause them. So can hypothyroidism, which tends to make nails thick and brittle along with the ridging. Iron deficiency is another possible cause, sometimes paired with nails that curve inward like a spoon. If your ridges appeared suddenly, look dramatically different from nail to nail, or come with other nail changes like brittleness or discoloration, it’s worth mentioning to your doctor. Otherwise, they’re cosmetic.
Horizontal Grooves You Can Feel
Horizontal indentations that run across the nail, known as Beau’s lines, are different from vertical ridges in an important way: they usually mean something interrupted your nail’s growth temporarily. You can feel them when you run a finger across the nail surface. They look like a dent or groove pressed into the nail plate.
The triggers are almost always systemic, meaning they affect your whole body. Common causes include high fevers, severe infections (including COVID-19), malnutrition, and chemotherapy. Localized trauma to the nail can also cause a groove on just one finger. Because nails grow at roughly 3.5 millimeters per month, you can estimate when the disruption happened by measuring how far the line is from your cuticle. A groove sitting about 7 millimeters from the base likely formed around two months ago.
Beau’s lines grow out with the nail and eventually disappear on their own. If you see them on multiple nails at the same position, that’s a strong sign your body went through a stressful event, whether you realized it or not.
White Horizontal Lines
White lines running horizontally across the nail come in a few varieties, and telling them apart is straightforward once you know what to look for.
One type, called Muehrcke lines, appears as pairs of white horizontal bands. They’re smooth to the touch, they disappear when you press on the nail, and they don’t grow out as your nail grows. That last detail is the key distinction: these lines are in the nail bed underneath, not in the nail plate itself. They’re most commonly linked to low albumin, a protein made by your liver. Conditions that lower albumin levels, like liver disease, kidney disease, or malnutrition, can produce them.
Another type, called Mees’ lines, looks similar but does grow out with the nail and feels smooth. These are associated with heavy metal exposure, poisoning, or severe systemic illness. If you notice white horizontal bands on several nails, pay attention to whether they move as the nail grows or stay in place. That distinction helps identify the cause.
Dark or Pigmented Vertical Streaks
A dark brown or black line running vertically down your nail deserves closer attention. This is called longitudinal melanonychia. It’s caused by melanin-producing cells in the nail matrix becoming more active, and the reasons range from completely benign to potentially serious.
In people with darker skin tones, including those of African, Asian, and Native American heritage, pigmented nail bands are common and often harmless. They can also result from minor trauma, fungal infections, or certain medications. However, a dark vertical streak can in rare cases be subungual melanoma, a type of skin cancer under the nail.
Dermatologists use a set of warning signs to evaluate these streaks:
- Age and ancestry: highest risk between ages 50 and 70, and in people of African, Japanese, Chinese, or Native American descent
- Band appearance: brown to black, wider than 3 millimeters, with irregular or blurred borders
- Change over time: rapid growth in width or darkening
- Digit: the thumb, big toe, and index finger carry the highest risk, especially on your dominant hand
- Extension beyond the nail: pigment spreading into the cuticle or surrounding skin (called the Hutchinson sign) is a significant red flag
- Family history: personal or family history of melanoma or atypical moles
A single dark line that stays the same width and color for months is less concerning than one that’s changing. But any new dark streak on a single nail, particularly if it appeared in adulthood, is worth having a dermatologist evaluate.
Tiny Dark Lines Under the Nail
Thin, short, dark lines that look like splinters trapped under the nail are called splinter hemorrhages. They’re tiny streaks of blood from damaged capillaries in the nail bed, and they run vertically along the direction of nail growth.
The most common cause is simple trauma. Bumping or jamming your finger can break small blood vessels, leaving these marks. They’re also seen in people who do repetitive work with their hands. In rare cases, splinter hemorrhages on multiple nails, especially near the base of the nail rather than the tip, can be associated with endocarditis (an infection of the heart valves) or other vascular conditions. A few splinter hemorrhages near your fingertips after physical activity are nothing to worry about. Multiple ones appearing across several nails without an obvious cause are worth investigating.
Nails That Are Half White, Half Pink
Sometimes the line on your nail isn’t a thin stripe but a broad division of color. Two patterns are worth knowing about.
In one pattern, the bottom 80% of the nail appears white while just a narrow pink or brown band remains at the tip. This is associated with liver cirrhosis, congestive heart failure, and diabetes. In the other pattern, known as half-and-half nails, the lower portion is white and the upper 20% to 60% is reddish-brown. That distal band doesn’t fade when you press on it. Half-and-half nails appear in up to 40% of people with chronic kidney disease.
Both of these patterns tend to show up on multiple nails at once. They’re not something most people notice on their own unless they’re looking for it, and they’re typically found alongside other symptoms of the underlying condition.
How to Read Your Nail’s Timeline
Your nails are a slow-motion record of your health. Fingernails grow at an average of about 3.5 millimeters per month, while toenails are slower at roughly 1.6 millimeters per month. The pinky fingernail grows the slowest of all the fingers.
This growth rate means a horizontal groove or white line halfway up your fingernail probably corresponds to something that happened two to three months ago. If you had a high fever, a surgery, or a period of severe stress around that time, the connection is likely straightforward. Lines closer to the cuticle reflect more recent events, while those near the tip are older. Once the affected section of nail grows past the fingertip, you can trim it away and the nail will look normal again, assuming the underlying cause has resolved.