If I'm right this is one of Zara's Ascot-hats. I wonder if it's Philip Treacy? Lovely hat, bright and soft colours suit always good to her. But I didn't like it so much together with the sunglasses (although the sunglasses per se are quite nice too).
I've rarely seen Mary with hats that wouldn't have been gorgeous.
She has a wonderful taste in hats and so is this one here.
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Please excuse my english and correct me, if I'm wrong.
There's a lady who's sure all that glitters is gold...