Put simply, a lock is a fastening device of some sort that prevents access to a certain resource unless the appropriate key (be it physical or informational) is possessed. Locks have existed for thousands of years, and perform vital roles in most societies, both in preserving privacy, and protecting property. Without locks our houses could be burgled with ease, and security would have to rely on expensive measures such as armed guards. Recently, the concept of the lock has been applied to digital files, preventing access to those without the appropriate key, but we may consider this more a subset of cryptography than as a technically related, despite the similar functions.
There are two main ways in which locks fulfil their function. The first of these is that of strength. Locks are usually made of metal, and designed so that they cannot be easily broken with force. However, in most cases, any sufficiently dedicated criminal can eventually damage the lock, and bypass it. It is here that the symbolic aspect of the lock is much more important. The lock signifies that any person found tampering with it is up to no good, as any legitimate user would have the key. Trying to break a lock will cause noise and attract attention, so the presence of even a fairly flimsy lock in a public place will act as a significant deterrent. In addition to this, a broken lock is a clear sign that unlawful access has been gained, and that anything protected by the lock can no longer be considered a secret. This closely parallels the use of sealing wax on letters, to prove to the recipient that the letter had not been read by anyone else.
Many years ago there were no locks in any recognisable form. To make possessions more inaccessible they were often held by elaborately tied knots in rope, the most famous example of which was tied by King Gordius of Phrygia. However, as Alexander the Great demonstrated, such an object is easily destroyed through force. The only lasting use of the rope lock was in knots such as the thief knot, which looks very much like a reef knot, but can be distinguished by careful inspection. A careless thief might switch the coins in a merchant's pouch for forgeries, but would probably retie the pouch with a reef knot, leaving clear evidence of his doings.
The earliest mechanical lock found comes from the region of Nineveh, but we have many more examples of this design from both the Middle East, and areas as far flung as Scandinavia and Egypt. This was actually more similar to modern locks than many later examples, as it consisted of a number of wooden pins concealed within a box, that could only be lifted from the bar they were locking by the application of a key with projections in the right places. Early locks such as this were usually of all-wooden construction, and thus were not very strong.
The Romans and Greeks created lock designs that were more robust, but simpler, as they worked on the principle of the latch, which could usually only be opened from one side of a door. By encapsulating this within a casing, and leaving a hole, the latch could only be operated by those with a correctly shaped key to reach inside the hole and lift the latch.
While we are unsure of the origin of the padlock, it seems likely that due to its prevalence in the far east that this it was developed in this region. Primitive padlocks operated on a similar principle to other early locks, but instead of holding a bolt on a door in place, the pin, or pins secured the other end of a loop that originated from the padlock. Locks such as this were popular due to their portability and versatility.
Pins, Tumblers and Iron
Gradual improvements in lockmaking technology led to stronger, more reliable locks that were less easy to pick. It is thought that most developments in lockmaking technology that led to a lock more like that we would recognise today took place in Europe, where it was realised that steps needed to be taken to prevent similar keys from being able to open the same locks.
The first innovation to prevent this was a refinement of the Roman invention of "warded" locks. Various pieces of metal were incorporated into the locks, that would get in the way of any key that wasn't of the right shape, and prevent it from rotating. Due to the advances in metallurgy, keys and locks could become more complicated than before, and so security was enhanced. Wards became increasingly complicated, with wealthy families being prepared to pay large sums of money to locksmiths that would design a lock with a key in the shape of their coat of arms. Many church locks from between the 14th and 17th centuries have keys in symbolic shapes, with cross motifs and other embellishments. Unfortunately, wards were not the end of the competition between locksmith and lockpicker, as the shape of the ward could be quite easily distinguished in most cases by putting a wax pad into the lock, and pressing it against the ward. This would enable the fabrication of a key that would not be blocked by the wards, and would thus be able to turn in the lock, and release the bolt.
Unfortunately, there were few other significant developments in the medieval period, despite the efforts of craftsmen to make their locks more elaborate and safer. Most "inventions" of this period were not related to the actual mechanism of the lock, but were various means of preventing access to the keyhole, or hiding the lock mechanism. These did nothing to make the lock more difficult to pick, but just increased the amount of knowledge necessary to operate it. A good example of this is the mechanism on one of the side doors of Winchester Cathedral. The lock is a simple warded bolt, and there is a handle above this to operate a latch. However, the handle is not connected to the latch, instead you have to push aside one of the studs next to it.
One of the most important inventions of the Renaissance was the lever tumbler, first created in England by Robert Barron. This was a device that held the bolt of the lock in place unless there was a key in the hole, that caused the tumbler to be held in the correct location, no longer stopping the bolt's rotation. It is this concept that is vital to modern locks, but at the time the importance was not realised.
Most modern locks are secure because they have a large number of tumblers, which must all be held in the right positions before the key can cause the bolt to rotate. It is this combination of the concepts used in the earliest locks, with the application of precise metalwork, and sliding bolts that made locks both reliable and strong. It is because of these many tumblers that keys for this sort of lock have a jagged appearance, as each projection is the correct height to hold one of the tumblers in place.
The development of the combination lock was a direct result of the increasing use of medieval trick locks. Some locks had dials incorporated that had to be set to the correct position to allow the key to enter the lock, or turn the mechanism. As more and more dials were added, in some cases the key was dispensed with entirely, and opening the lock simply required all the dials to be correctly set. Combination locks also had the advantage that the mechanism could be well armoured, as there needed to be no keyhole, which was especially important following the invention of explosives.
Towards the Modern Day
Lock design became more and more complex, and locksmiths became competitive about the security of their locks. For example, in 1784, Joseph Bramah created a competition, offering 200 guineas to anyone who could pick his lock. This puzzle was unsolved more more than 50 years until, after more than 2 days of continuous work, an American locksmith succeeded in 1854.
It was during the 19th century that lock manufacture was revolutionised by the advent of mass production. Until that point locks had been expensive, and were painstakingly produced by hand. Before 1800 Europe was the centre of the lock making trade, both in terms of quantity of locks produced, and in innovation, but America increasingly began to produce its own locks, and many new designs were patented there. While some of these have become widely adopted, the basic mechanism by which most locks operate has not changed for several hundred years.
Linus Yale Jr. was one of the most important innovators of the 19th century. He patented many designs for stronger locks that were less easy to bypass with force. Although most of his inventions were concerned with making stronger locks for bank vaults, it is the business that he set up in 1868 for which he is still remembered today. Other important inventions of the period included such devices as the unit lock, which instead of affixing to the outside of the door, as with all previous locks, could be installed within a slot in the edge or the door. This design is used on most internal locks today. Other famous names within lock design, such as Jeremiah Chubb were also active during the 19th century.
Further advances in lock design occured through the early parts of the 20th century. One important example of these was the responsibiltiy of Walter Schlage, who, during the 1920s created the first lock that was constructed within a door knob, now a standard design. Another major innovator of this period was Harry Soref, who founded the Master Lock Company. He applied the principle of lamination to lock design, and created a very strong lock casing made out of many layers of steel.
While locks of increasing complexity have become available since the first half of the 20th century, by that point all the major principles were in place, and innovations in lock design have generally been linked to new materials, or improvements in manufacturing techniques. You might think that locks nowadays must be very secure, but while such locks are available, they are not the norm. With no formal training I have been able to pick the majority of padlocks I have tried within 30 seconds, sometimes almost instananeously. For security, you should get a lever tumbler lock with a deadbolt.
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