England is old and small and the local folks started running out of places to bury people.
So they would dig up coffins and would take the bones to a bone-house, and reuse the grave. When reopening these coffins, 1 out of 25 coffins were found to have scratch marks on the inside and they realized they had been burying people alive. So they would tie a string on the wrist of the corpse, lead it through the coffin and up through the ground and tie it to a bell.
Someone would have to sit out in the graveyard all night (the graveyard shift.) to listen for the bell; thus, someone could be, saved by the bell.
Or was considered a.. dead ringer..