Of course Pickety doesn't mention it. That would ruin his narrative. And it wouldn't back his central thesis and proposal to tax the living excrement out of the rich. You know, to fix a problem none of us ever knew of until he "told us" about it in his giant weary book.
But I'll hold off on a proper rant about him and his book once I've read it.
Slightly more than 100 years ago, all the major currencies of the world were backed by precious metals such as gold; now, none of them are.