That's a bit like saying whisky brewed in Japan is Scotch.Marans bred to the French Standard do. Those bred to the UK Standard are clean-legged.
These are all clean-legged Marans I had back when I was fifteen or sixteen and both varieties had been imported into the US.
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I did have bloodlines with feathered legs also. But a lack of foot feathers doesn't mean the bird can't be a Marans. Especially in the US where backyard breeders/ignorance runs rampant and people breed anything to anything to try and sell them. How many Marans hybrids are available from the large hatcheries now? Also, there are Black Marans. Just because they aren't Black Copper Marans doesn't mean they can't be Marans.