I get thin, short, black vertical lines beneath my fingernails that grow out. months will pass and no lines and then they appear again. what are they?

Splinter bleeds?: If you're describing splinter hemorrhages (there many web photos for you to compare), you'd do well to show them to your physician, though this isn't the classic presentation that raises concerns. I always have splinters just from cutting my nails super-short. You may be describing something else, and again i'd urge you to have them seen.

Nail anomalies : Such discolorations can be caused by intermittent trauma to the nail unit from biting or banging , etc. Other causes could be low-grade infection or inflammation.