Quite recently (it can still be seen in the list of new tags) the tag line was created. (As far as I can say, it was created in this post. Or at least it was the first post using this tag that I saw. It was later removed from that post, but added to other posts before it was removed from the system, see also here.)
There are several natural questions:
- Should we keep the tag? Will it be useful?
- What should be content of the tag? (The tag-info is empty at the moment.)
- Most tags are in plural. Should lines be added instead or at least as a synonym?
- Should we add similar tag for planes?
We have many tags about specific geometric objects: circle, triangle, quadrilateral, conic-sections, spheres, polyhedra, ... Lines and planes are definitely important in geometry, too. I can imagine this tag being useful, for example, for many questions in analytic-geometry. Many questions about them can appear also in linear-algebra and affine-geometry. But it would definitely be useful to clarify the usage of the tag at least a bit. (At least if we want to keep this tag.)
I am not very active in geometry-related topics. So I suppose that opinion of users who are interested in geometry questions and follow such questions on this site would be a great input into the discussion about these tags.