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In my mind I always end up trying to create a binary rating system when thinking about rating images, which is a strangely frequent occurrence in my mind...

The easiest would be safe and not-safe, but you could flip that I suppose. From a work-safe point of view the last bit wouldn't be a good idea. Shortly after coming to that conclusion I go "but three ratings works fine too and it allows for easier blacklisting and what not so it's al right" and I'm back where I started.

But aside from the broken inner workings of my brain, I suppose the main thing in rating an image is that the rules for rating need to be clear and easy and preferably short. No exceptions, no frills, but pure crystal clarity. So what is this mystical boundary...

Well an easy one would be as follows and I think it would work:

Explicit : Exposed genitalia.

Questionable : Visible nipples, areola etc, visible facsimiles of genitalia, covered visible genitalia.

Safe : Everything else.

Elaboration

It leaves some gaps, suggestive acts for one. You can fix that either with the already used general tags for the acts or you can choose to not fix it and let the viewer deal with what is effectively whatever the hell they think it is.

Guro is another 'problem area' I suppose, but a lot of the stuff that really wants non-safe ratings is usually questionable anyway...

Another solution there would be a quick list of tags that warrant a certain rating, but that would add clutter to clarity. I'd keep that list short, very short. Preferably length 0.

That leaves one thing bugging me to no end. The social acceptance of the unclad male torso that the unclad female torso does not have. The one main problem, a beach scene would be questionable by default with this system if it had male characters in it. That's pretty fair and I could live with it, but to avoid changing much from current rating, rate q only on female nipples, the rest would stay the same.

So to answer the second post, a safe breast would be questionable when it is
1. Exposed down or up to the nipple
2. Female

P.S.