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Nice catch about the primary tag being included in the URL! Although that's not true for all interfaces, but still great for SEO, since I was talking about that yesterday.

Also the primary tag is the only one displayed on the posts thumbnails, on some interfaces.

Yeah I know, and I am not a fan of that behavior, I want to set the tags myself.

Ah, you're talking about interfaces which set their own tags as primary tags, if they are not already added. I would make this optional, or at least not set them as primary tags unless they were set by the author.

I was referring to something of visual nature. That the primary tag is the only one displayed on the the post thumbnail, in your feed, on most interfaces.