Recently we've had a spate of new tags that are awkwardly named, redundant with another tag, or don't adequately justify their existence. Seemingly every time, veteran users tell the tags' creators they should have trialed the new tag as a set, but the tag creator says they didn't know that was an option. Their ignorance is understandable, given this supposedly common knowledge is not mentioned in e621:Tagging Checklist or any other help pages I could find. (The Tagging Checklist does mention the existence of sets, but only recommends using them for subjective concepts, not new tags.)
I'd like to suggest this advice be added somewhere new users are likely to see it. It could be added to the Tagging Checklist, or maybe even the upload form. While it's true that most new users will likely gloss over it anyway, it would curb this problem at least a little.
(A link to the "Do we have a tag for that?" thread would be a good addition, too.)
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