The security coffin designed by Dr Johann Gottfried Taberger in 1829 alerted a cemetery night watchman by a bell which was activated by a rope connected to strings attached to the hands, feet and head of the ‘corpse’. The bell housing prevented the alarm from sounding by wind or birds landing on it.
This phrase supposedly dates back to a time when people were at risk of being buried alive. To keep from waking up inside a coffin (and then really dying), loved ones were buried with bell ropes so they could ring the bell if they woke up. Once someone heard them, they were dug up and thus “saved by the bell.”
Also known for the phrase “Dead Ringer!”