The first one looked like a stubby garden rake. With a long handle to reach through a hole in the door and a row of pegs to push up bars which locked the door, this system was the rustic forerunner of the modern "pin tumbler" lock. Though it originated in Egypt and Persia 2,500 years ago, it was still in use in remote communities, such as the Faroe Islands, a century ago.
The Romans invented the portable spring-loaded clasp that we know as the padlock. But the superior tumbler mechanism was virtually ignored until 1848 when an American, Linus Yale, introduced the serrated keys and crooked keyholes that now give access to front doors everywhere. His design allowed for thousands of permutations while performing the simple task of lifting five pins to precisely gauged settings, thus enbling the barrel of the lock to turn.
In the Middle Ages locksmiths preferred wards - fixed obstructions that prevent the key from turning - which resulted in large, elaborate-looking keys and locks but didn't do much for medieval crime prevention because they were easy to pick. Keys became a status symbol - an obvious indicator that you had something worth stealing - and the custom began of presenting the keys of the city to honoured citizens.
Then, in 1818, Jeremiah Chubb of Portsmouth patented the detector lock, which would seize up if the tumblers were moved too far by clumsy attempts to pick it. The wealth and property boom of the industrial revolution heralded a golden age for lockmakers - and pickers. But, despite the promise of a pound;100 reward and a pardon, a notorious convicted lockpicker spent several months trying, and failing, to open Mr Chubb's invention. Today, swipe cards have replaced keys as the means of entry to many office buildings. But that familiar jingle jangle - signal of approaching caretakers and prison warders alike - is a reminder of their reassuring presence. Just don't leave home without them.