Iron was the most common material for locks made in medieval Europe because it was believed to have the power to ward off evil spirits that might try to enter homes or churches through keyholes. Locks often were forged with images of a dragon's head on them, which was meant to scare away demons.

Around the 16th century, lock design became less about function or superstition and more about decor, thanks to the flamboyant and frilly baroque and rococo styles introduced in France. The throne demanded the most talented craftsmen for every detail of the palaces, right down to the door knobs, door plates, locks, and keys. European padlocks, too, became much more fanciful during this era, shaped like hearts, triangles, shields, or barrels, featuring spring locks and swinging shackles that swooped and scrolled like cat tails.

“Pull locks” were employed for sheds and barns where valuable stores of food and grain were kept on farms in the 17th century. To open such a lock, a person had to manipulate the key with one hand while pulling the ring attached to the bolt with the other.

In the early 18th century, Swedish mechanical engineer Christopher Polhem created his “unpickable” padlock, the forebear of the modern padlock. Inside the lock was a series of stacked metal disks. The key could only be removed when the disks were aligned in the “locked” position. These padlocks are known today as Polhem or Scandinavian locks.

During the late 18th and early 19th centuries, the English made locks that were difficult to pick thanks to complicated lever systems—examples include Robert Barron’s 1778 double-acting tumbler lock and Jeremiah Chubb’s 1818 lock. In 1784, Joseph Bramah patented his safety lock, which employed a cylindrical key with slides that must be cut to the correct length to turn the bolt.

The Victorian Era was the height of invention and creativity when it came to making padlocks in the United States and Europe. Barron's lever-tumbler invention led to padlock manufacturers boasting the number of levers their locks contained, sometimes even marked on the lock like the Cleveland 4-Way. Some padlocks contained as many as 6 to 8 levers; the more levers, the more secure the bolt.

These antique padlocks usually featured brass levers, which doesn't rust easily, while the casings could be made of brass, tin, steel, or iron. In a frenzy to patent their lock designs, makers created beautiful logos with words, pictures, scrolling, or monograms cast into them—once patented, no other company could use those words or images in their locks.

Some of the most stunning 19th-century locks feature bull dogs, lions, or dragons on them. Highly collected brands from this era include Winchester, Sure Grip, WB (Wilson Bohannan), Yale, Corbin Ironclad, Miller, and Excelsior. Even the famous E.C. Simmons Keen Kutter brand had a lock in its trademark pie-and-wedge logo shape.

Push-key lever locks, particularly flat ones called "pancake locks," are especially favored by collectors. Commemorative locks, too, have been issued for big events in the United States, and also for special anniversaries of lock companies.

Even more coveted, though, are story locks, which feature elaborate pictures on both sides. These rare padlocks were made in the late 1800s and would have, for example, the face of a man leaning through a window on the front, his mouth as the keyhole, and his legs sticking out the window on the back, with perhaps a dog tugging at his ankle. Collectors have to be wary, though, as some story locks have been reproduced.

"Logo locks" are those made with the name of the end-user company on them as opposed to the lock manufacturer. The first of these were made for railroads, a kind of lock particularly coveted by railroadiana collectors. Many of these are brass and heart-shaped, but they came in a wide variety of shapes and sizes. The only defining feature of the railroad lock is that it features the name or initials of the railroad company, from the Northern Pacific to the Santa Fe.

Starting in 1925, the Best Universal Lock Company capitalized on the logo-lock concept with its interchangeable core locks. Since its founding, Best has emblazoned the names and logos of more than 5,000 end-user companies on its plain-looking brass pin-tumbler locks. Logo locks generally found their way to the public through employees of the end-user companies, who received old locks as souvenirs.

Padlock collectors sometimes specialize in locks made for the U.S. military, or those used by the U.S Postal Service. Others are fond of miniature locks. Two especially intriguing categories of padlock-collecting are cut-away locks and trick locks. Cut-away locks are generally locks designed by hardware manufacturers with pieces cut out to show off the lock's inner workings. These were usually used as salesmen samples and in stores to demonstrate how the locks work, although some were made by tinkerers and educators to study locking devices.

Trick locks have been around since the early days of warded locks. Since warded locks were so easy to pick, trick locks also required secret knowledge to spring the lock. You'd have to know to push, pull, or slide special buttons or levers in certain sequences to even reveal the keyhole. Some of these puzzlers require using various keys in a particular order.

Many antique padlocks have corroded completely, while others were melted down in World War II scrap-metal drives, creating a tantalizing scarcity. Padlock collectors should know that it is pretty rare to find an antique lock together with its key, and you won't necessarily be able to get an accurate key made for an antique lock.

Outside of padlocks, some collectors specialize in prison locks, which tend to hit the market when a penitentiary replaces its locking devices. These jail locks are large and heavy, their casings made out of dense metals like iron or steel, their tumblers made from rust-resistant brass. Most of antique prison locks contain a lever mechanism. High-security prisons still use lever locks, but since the mid-20th century, U.S. jails have also been using pin-tumbler locks. While most cells use the same key, the keys for such antique locks (often double-bit spike keys) are rarely found.

It wasn’t until the 19th century that mortise door locks, or locks that are contained inside the door itself, were invented in England. Before the 1830s, all lockable exterior and interior doors used rim locks, which are mounted on the outside of the door. The mortise lock became the dominate form of lock for the next 100 years.

At first, American mortise locks had plain brass doorplates, but as Victorian decorative hardware became all the rage—in revival styles ranging from Spanish, Colonial, Tudor, Gothic, Italianate, Queen Anne, Greek, Flemish Renaissance, and Japanese—doorplates, keys, strike plates, and lock sets became more elaborate.

Antique interior mortise locks are not especially popular with collectors since they are often incompatible with contemporary doors. The doorplates, and in particular keyhole escutcheons, however, are well-loved. These come in a wild variety of shapes and designs, some with double keyholes, others with swinging covers often featuring animal designs.

Finally, in 1848, Linus Yale and his son began to develop the cylinder lock, also known as the modern pin-tumbler lock, which is the lock that’s most commonly used today in doors and padlocks. The key has a blade instead of a shank, and the blade is cut with horizontal grooves that allow the key to be inserted past the wards in the cylinder. Then, when the key is turned, the “bittings” carved along the bottom of the blade operate the pins or wafers inside the lock, allowing the cylinder to turn freely.

Bank robbers found safes with key locks relatively easy to crack. In 1861, Linus Yale, Jr., introduced the first modern combination lock, which used a combination of numbers, instead of a key, to open the locking device. This concept was quickly adapted for bank locks—nearly as quickly, resourceful thieves figured out a way to crack such locks. Safes and bank-vault locks are also popular with collectors.

James Sargent, an employee of Yale’s, added an improvement to the combination lock when he developed his “theft-proof lock,” which added a timer to the device so it could only be opened during certain hours. In response, bank robbers turned to explosives to open bank safes. Sargent went on to form his own esteemed lock company.

In the 1920s, hardware makers began to reconsider door hardware. With an old mortise-style lock—which are still seen in old houses today—the locking mechanism is a separate device from the door knob that turns the latch. The two are connected by a spindle inside the mortise lock set installed on the door. This allows the lock, operated a by a key usually below the door knob, to hold the knob and latch in place or release them so the knob turns freely.

In an effort to improve upon this technology, manufacturers figured out a way to integrate a cylinder lock inside the door knob, making the knob and lock one device and paving the way for the modern cylinder lock sets we use today.

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