safety chain

noun
  1. a chain on the fastening of a bracelet, watch, etc, to ensure that it cannot open enough to fall off accidentallyAlso called: guard
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012