In a French link bit, the center link is rounded, shaped, and designed to lie smoothly on the horse's tongue.
A correctly-designed Dr. Bristol (and many are NOT well-made, which adds to the confusion) has a center link that is a long, thin plate set at a slight angle. A proper Dr. Bristol is designed to be used in TWO ways -- it can be put into the horse's mouth so that the plate lies flat and works almost like a French-link, OR it can be put into the horse's mouth so that the plate lies at about a 45-degree angle. This is when it is severe.
(from Jessica Janiel, but I've read it in other sources too back when I was looking for bit for my horses).
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