What causes grooves in (big) toenails?

Factual Questions
1

Every so often I get kind of a groove/indentation in one of my toenails, of my big toes. They’re perpendicular to the toes, and as the nail grows out they make their way along until they get clipped.

Question is what causes such a thing. I’ve never had such a thing on a fingernail (and don’t recall any on any toes other than big toes).

2

Vertical lines are a symptom of age, dry skin, some vitamin deficiencies.

Horizontal lines, like you are seeing, are called Beau’s Lines and could be an indication of something more serious.

3

That’s interesting and helpful. Thanks!