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You asked 'Why upside down?' Boo. Well, I once read that if you are painting from a photograph or doing a portrait then it is often easier if you turn the photo upside down. Your brain has to re-register the image, - it isn't a photo of a cat (or person etc) now, it is just a series of shapes. That way, you can draw what you really see, and it's easier to get the proportions right. It does seem to work.
I wonder... has anyone else tried this?