And Lee Steffen has figured out what they are. Here are the first two types on his list. You can find the rest on his Twitter feed.
1. Orange and Blue Action
The responses to Mr Steffen’s list are also worth a look. One commenter, for instance, pointed out that Benedict Cumberbatch isn’t allowed to face the camera on movie posters.
Another found this interesting case of twinning.
Much more at the site.