Every once in a while I discover myself in the need of improving the placement of the tombstone symbol such as ∎ or □ at the end of various environments. (The latest example is QED for cases and tables in the presence of margin notes (and todonotes), and I am very likely to ask more.) Some other folks wish to color the symbol, use two different Halmos symbols in the same document simultaneously, and so on. I was surprised to find out that no tag exists for the topics around the Q.E.D. symbol, intentionally vaguely speaking.

So, I'm wondering whether the community is likely to welcome or reject this new tag. In case of acceptance: should old questions concerning QED (652 as of now) be considered as candidates for re-tagging?

As for naming, my personal opinion would be not to choose QED/Q.E.D./qed (quod erat demonstrandum, "what was to be demonstrated"), due to the various usages (e.g., example end, definition end, the end of the statement of a lemma without a proof) and various symbols (including, e.g., ◊ and ⧫, as well as those suggested by Wikipedia). Something else, say, "end-of-environment-symbol"/"end-of-theorem-symbol"/"tombstone"/"tombstone-symbol"/"halmos"..., could be more appropriate.

As for the suggestion to use the already established tag {theorems}, some folks have tried to remove this tag from QED for tabbing +SVMono +todonotes +geometry +newtx -ntheorem -amsthm just right now. This is probably due to the fact that the MWE there has no theorems at all. So, there is at least one person except me who thinks that {theorems} might be not appropriate.

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    Just use the theorems tag. I don't think there is a need for such a specific tag. – Henri Menke Jun 7 '18 at 22:40
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    No idea what 'tombstone' refers to. Know quite well what 'QED' refers to. It is not an end-of-environment symbol. You don't use it at the end of a list or tikzpicture and I don't use it at the end of isdrafod. Why should the fact that people use different symbols for QED make the 'QED' tag inappropriate? I don't really see why we need such a specific tag but, if we do, please leave it as something people can actually understand. We have enough incomprehensible tag names as it is. – cfr Jun 8 '18 at 0:17
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    It doesn't look anything like a tombstone. Why isn't it curved on top? How many graves have square tombstones? Non-square rectangles I've seen, curved rectangles, crosses, statues, but not squares. – cfr Jun 8 '18 at 0:19
  • @cfr It looks like one, in the context of typography (see here). But of course a new user is unlikely to recognize it. – TeXnician Jun 8 '18 at 5:53
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    I have no strong opinion about whether or not such a tag should be created. I'd have thought it would already exists and there are quite a lot of questions about QED marks, so why not? On the other hand even the forum search finds "QED" easily and one might feel that adding more and more tags makes it harder to actually select - at most - five relevant tags for a question. That said, I strongly favour qed over tombstone. The search shows a 650:50 ratio in favour of qed and while "QED" might not be used in all languages it is used in command names involving the symbol ... – moewe Jun 8 '18 at 6:23
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    ... and is probably known to most mathematicians with a command of the English language that allows them to write and read posts on this site. Tombstone has other meanings associated with it and the typography usage is definitely niche (I found nothing in various bilingual online dictionaries even the online OED had nothing). The tag wiki could of course mention tombstone, but I think the primary tag should be "qed". – moewe Jun 8 '18 at 6:32
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    I'd also strongly favour qed over tombstone. Besides the fact that qed feels more intuitive to me, a tombstone tag is much more at risk to be "taken over" by other questions, e.g. tex.stackexchange.com/q/181366/36296 – user36296 Jun 8 '18 at 9:31
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    Having a look at the search results tex.stackexchange.com/search?q=tombstone many (most?) of the posts mention qed in the title and tombstone only in the body of the post. – user36296 Jun 8 '18 at 9:47
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    @moewe ... or Latin, I assume :-). – cfr Jun 9 '18 at 2:33

Certainly if a tag is added qed should be the tag name rather than tombstone, but I think the existing theorems tag should be fine, although several questions asking about this do not currently have the theorem tag and could have that tag added

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    @MdAyquassar well you'd have to give up one of those tags to add qed as well, so i don't see that makes much difference. – David Carlisle Jun 8 '18 at 12:55
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    @MdAyquassar and as I said on your previous question the tags are really of so little importance if you deleted a couple and added theorems instead it could not possibly do harm. tags are important on the major stackoverflow site as they basically work like sub-sites, if you filter on the tag xml you see different questions and different regular people than a tag like python, but here it isn't that way all the questions are interlinked and the tags just form a slightly strange search aid, and a badge making mechanism. – David Carlisle Jun 8 '18 at 12:57
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    Completely agree with the answer. Only partially agree with second of your two comments. I think tags do matter a bit. I use them to filter questions, for example. If I have lots of time, I might look through the unfiltered list, too, but only if I've not found something interesting in the filtered lists. However, this may depend a lot on what you're most interested in, so filtering by some tags might be more important than others. Absolutely don't see how having to choose 5 of 7 tags would be better than having to choose 5 of 6, though. – cfr Jun 9 '18 at 2:30

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