I just invented the schengen-d-visas tag since I noticed that visas of this type were often enough referred to to warrant a tag of their own.
But as I got to the end of the list of questions with both "visa" and "D" in them, I realized that a previous tag, long-stay-visas, already exists for the same job.
Since so many newer questions don't have the old tag that shows it's not obvious to everybody so having both synonyms should make it more discoverable.
So please turn my new tag into a synonym of the old tag.
Or if enough of you go to the tag synonym page and vote for it as a synonym that ought to have the same effect.