The Curb chain is important on all bits that need a  Chain if you do not use a Chain on a Pelham or Weymouth then that unbalances the bit causing the cheek to rotate too far over and then the Bit applies too much mouth pressure and too much poll pressure  with no balance from the Chain.

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 It should lie flat against the Horse’s jaw and the little fly link should be at the bottom It should be set so  as the cheek of the bit turns to about a 45% angle the chain should tuck snuggly into the jaw that stops the cheek rotating any further over

Too tight and the Horse will get no relief from the pressure and too loose and the bit will rotate too far in the Horse’s mouth.