What are these thin black lines under the nail?

I’ve recently come across a client who has the smallest splinter looking like a black tiny thin line under the nail. Haven’t come across this before. What would/could this be?

We often see lines like that under our clients’ nails. They are called xuất huyết dằm và về cơ bản là những đốm máu nhỏ do chảy máu dưới móng tay.

There is no physical pain or discomfort associated with the hemorrhages unless it has been caused by physical trauma like a bang to the natural nail. Usually, they will grow out along with the nail and within three or four months the hemorrhage lines are no longer seen.