You've all heard the old expression "Mad as a hatter", and might have wondered what that means. Quite simply, the manufacturers of hats once used mercury in the manufacture and manipulation of felt hats. Mercury is, of course, quite toxic and the repeated exposure to the metal caused the hat maker to become quite mad.
When you look at a good, felt hat, you will notice that it has a leather sweatband, and at the back of the hatband is a little ribbon bow. Many have wondered why that bow is there, and the history of it is sketchy. Some say that originally it was a sizing device. Indeed, it may have been a sizing apparatus. Others say that the little bow was simply a device where the hatter could feel inside the hat and know where the center of the back of the hat was, so that he might correctly place the crease. Others say that the bow was for the wearer, so that the bow would go in the back, preventing the embarrassment of wearing the hat backwards.
The most intriguing story I've heard is that the little bow is a stylized representation of the skull- and-crossbones symbol for danger. It reminds the hatter that his is a dangerous profession and pays tribute to those who went before in the making of hats.
And now you know as much about that little bow as I do. If you've heard any other explanations, please shed some light in comments.