A line on your nail is usually a vertical ridge caused by aging or minor trauma, and in most cases it’s completely harmless. But nail lines come in several distinct varieties, and the type, color, direction, and number of affected nails all matter when figuring out the cause. Here’s how to tell what yours means.
Vertical Ridges: The Most Common Type
If you’re seeing faint lines running from the base of your nail to the tip, like tiny grooves in the surface, these are longitudinal ridges. They’re the single most common nail change associated with aging. In one study of older adults, 85% had prominent longitudinal ridges. The nail matrix, the tissue under your cuticle that produces the nail plate, gradually becomes less uniform as you age, creating these subtle furrows.
Vertical ridges on their own rarely signal a health problem. They tend to appear on multiple nails and become more noticeable over time. Iron deficiency can also cause vertical ridges, sometimes alongside spoon-shaped nails that curve inward. If your ridges appeared suddenly, affect only one nail, or come with nail brittleness and fatigue, a blood test can rule out a deficiency.
Dark Brown or Black Lines
A pigmented streak running vertically through your nail, brown to black in color, is called longitudinal melanonychia. This type of line gets the most attention because, rarely, it can indicate melanoma under the nail. But context matters enormously.
In people with darker skin tones, pigmented nail bands are extremely common and almost always benign. They appear in 77% to 100% of Black individuals, 10% to 20% of Asian and Japanese individuals, and about 1% of white individuals. Pregnancy, certain medications, and minor repetitive trauma (like nail biting) can also trigger these streaks by activating the pigment-producing cells in the nail matrix. When the cause is this type of cell activation rather than abnormal cell growth, the bands tend to appear on multiple nails and stay relatively stable over time.
When a Dark Line Needs Evaluation
Subungual melanoma, melanoma that starts under the nail, has a few distinguishing features. It typically affects only one finger, most often the thumb of the dominant hand. The pigmented band is wider than 3 mm, and it changes over time, often starting narrow and gradually broadening into a triangular shape that’s widest near the cuticle. One key warning sign is pigment that spills beyond the nail itself onto the surrounding skin of the cuticle or nail folds. Other red flags include nail cracking or distortion, pain, and bleeding.
Dermatologists use an ABCDEF checklist to evaluate these streaks: age and ancestry (more common in darker skin types, typically diagnosed between ages 20 and 90), band color and breadth (black or brown, wider than 3 mm), change over time, digit involved (thumb, dominant hand), extension of pigment onto surrounding skin, and family history of melanoma. A single dark streak that’s new, widening, or on just one nail is worth having a dermatologist examine, especially if you’re over 50.
Thin Red or Dark Lines Under the Nail
Tiny reddish-brown lines that look like splinters embedded under the nail are called splinter hemorrhages. They’re caused by damaged capillaries in the nail bed. The vast majority result from everyday trauma: bumping your hand, using tools, biting your nails, or even gripping a cane. Trauma-related splinter hemorrhages tend to appear near the tip of the nail.
When splinter hemorrhages show up closer to the cuticle end and appear on multiple nails without an obvious injury, they can point to systemic conditions, most notably infective endocarditis (an infection of the heart valves) or other vascular problems. A single splinter hemorrhage near the tip of one nail after you’ve been doing yard work is not a concern. Multiple ones appearing near the base of several nails, especially with fever or fatigue, is a different story.
Horizontal Lines and Grooves
A horizontal groove or indentation running across the nail, side to side, is called a Beau’s line. Unlike vertical ridges, which form gradually, Beau’s lines mark a specific moment when nail growth was temporarily disrupted. The nail matrix essentially paused or slowed down, leaving a physical dent in the nail plate as it grew out.
The most common triggers include chemotherapy drugs (accounting for about 36% of cases in one systematic review), other systemic illnesses (25%), physical trauma to the nail (12.5%), and infections (7.5%). High fevers, severe stress, and zinc deficiency can also cause them. Because fingernails grow roughly 3 to 4 millimeters per month, you can roughly estimate when the disruption happened by measuring how far the groove sits from the cuticle. A groove halfway up the nail likely formed two to three months ago.
Beau’s lines on just one nail usually point to local trauma. Grooves across all ten nails suggest something systemic, like a serious illness or medication side effect, that temporarily interrupted growth everywhere at once.
White Lines and Bands
White lines across the nail come in a few distinct forms, and a simple test helps tell them apart. Press gently on the nail and watch what happens.
- Mees’ lines are single white horizontal bands that don’t fade when you press on them. They form in the nail plate itself and slowly migrate toward the fingertip as the nail grows. They’re associated with arsenic poisoning, severe infections, kidney failure, and other systemic insults.
- Muehrcke’s lines are paired white bands that do fade when you press on them. They stay in the same spot on the nail regardless of growth, because the problem is in the nail bed underneath rather than the nail plate. They’re linked to low protein levels, often from liver or kidney disease.
Random white spots scattered across the nail are usually just the result of minor bumps or pressure on the nail matrix. They’re one of the most common nail findings in healthy people and grow out on their own.
Nails That Are Half White, Half Colored
If roughly the top portion of your nail looks pink, red, or brown while the bottom portion near the cuticle appears white, you may be looking at a pattern linked to organ disease.
Lindsay’s nails show a white proximal half with a reddish-brown distal band covering 20% to 60% of the nail. This pattern appears in up to 40% of people with chronic kidney disease. Terry’s nails are the opposite proportionally: about 80% of the nail bed appears white, with just a narrow pink or brown band at the tip. Terry’s nails are associated with liver cirrhosis, congestive heart failure, and diabetes. In both cases, the color change doesn’t fade when you press on it.
How to Tell What Your Line Means
Start by noting four things: the direction of the line (vertical or horizontal), its color (skin-toned ridges, dark pigment, red, or white), how many nails are affected, and whether it’s changing. Vertical ridges on multiple nails that have been there for years are almost certainly age-related. A single dark streak on one nail that’s getting wider needs professional evaluation. Horizontal grooves across all nails after a bad illness are your body’s record of a temporary slowdown, and they’ll grow out within six to nine months.
Nails grow slowly enough that changes you notice today often reflect something that happened weeks or months ago. That delay can make nail lines feel mysterious, but it also means most of them are already resolving by the time you spot them.