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I don't quite know what to call it or how to describe it, but my bathroom has a three-way mirror. That is, there is one large mirror over the washbasin and vanity, and one angled mirror on each side (the angled mirrors are actually the covers of the doors of two small medicine cabinets). These mirrors were in every unit and original to the building. A lot of people do away with them when they renovate their bathrooms. I kept mine for just that reason: you can look at yourself in one of the angled mirrors and see your back in the other angled mirror.
I mostly use them to check that my tie is under my collar on the back of my neck.

Not even by standing with your back to a big mirror and holding up a little mirror?

You must be very limber, I'm not able to see my back in a mirror at all.