What is 1 black line underneath your fingernail mean?

I have 1 black line underneath the fingernail on my thumb and forefinger of my right hand and 1 black line underneath the forefinger of my left hand. Where did these come from or what is the cause?

Does the black line run along the length of the nail rather than across? I assume it does and I believe that it's caused by a small release of blood under the nail, probably caused by a slight trauma to the nail. I don't think it's cause for concern and it will grow out as the nail grows.

I also have this black line running vertical across my nail. Its been here for about almost two years now on the same middle finger. I just want to know what excatly is it? and how to get rid of it? Because im fed up of people asking me "What is that on your nail!" And i have to make up a lie saying I got my finger stuck in a door cause idont know what this black line is. I had one before on my index finger on my left hand but it grew out in a couple months. But i find that this perticular black line on this nail Is not growing out anytime soon.

Kae -17yrz

Well same thing with me, would like to know what it is ofcourse, its on my thumb right in the middle and also verticle.

Mark - 19yo

Go to a dermatologist for a biopsy. It is often a mole, which could possibly be cancer.