Posted under General
I'd consider ignoring the usage of them by males when deciding between earring and earrings. They're more commonly depicted on females and frankly we have a hell of a lot more images of females than males on danbooru.
Not sure how accurate this is (due to the how I collected the data), but I'm up to that ~95% of danbooru's images depict female characters while only ~20% of images depict male characters.
It is also wise to consider how many "natural" ways that earrings can be obscured. Perspective can block one earring unless your viewing the face dead on. There is also hair to account for, it is assumed that more females have hair that extends beyond the earlobes. This does imply that earrings would be more appropriate.
Additionally, at least to me, there is usually a difference in meaning between wearing a single earring, paired earrings, and multiple earrings/piercings. I think having a separate tags for all 3 concepts has merit, but whether it's actually worthwhile or not is something else.
Someone already made a single_earring tag (only 2 posts though). I'd probably prefer that with an implication to earrings and just alias earring to earrings.
I'd be willing to go through the earrings tag and tag images of characters with one earring. single_earring of course it will have to be defined that it should be known the character is clearly wearing only one earring though, as eric pointed out perspective and such can cover them up.
There's no good reason to distinguish "singly visible earrings that are assumed to have a mate" from earrings, single_earring is fine for known singletons.
