A lock is a type of physical security device that can be used to delay or prevent a door, a window, a manhole, a filing cabinet drawer, or some other physical feature from being opened, moved, or operated. Locks typically operate by connecting two pieces together - such as by connecting a door to a door jamb or a manhole to its casement. Every lock has two modes - engaged (or "locked"), and disengaged (or "opened"). When a lock is disengaged, the asset on which the lock is installed can be accessed by anyone; but when the lock is engaged, only persons that can disengage the lock (through the use of a key, a combination, etc.) can gain access to the locked asset.

Locks are excellent security features because they have been designed to function in many ways and to work on many different types of assets. Locks can also provide different levels of security depending on how they are designed and implemented. The security provided by a lock is dependent on several factors, including its ability to withstand physical damage (i.e., can it be cut off, broken, or otherwise physically disabled) as well as its requirements for supervision or operation (i.e., combinations may need to be changed frequently so that they are not compromised and the locks remain secure). While there is no single definition of the "security" of a lock, locks are often described as minimum, medium, or maximum security. Minimum security locks are those that can be easily disengaged (or "picked") without the correct key or code, or those that can be disabled easily (such as small padlocks that can be cut with bolt cutters). Higher security locks are more complex and thus are more difficult to pick, or are sturdier and more resistant to physical damage.

Many locks, such as many door locks, only need to be unlocked from one side. For example, most door locks need a key to be unlocked only from the outside. A person opens such devices, called single-cylinder locks, from the inside by pushing a button or by turning a knob or handle. Double-cylinder locks require a key to be locked or unlocked from both sides.

There are many different types of locks that function in many different ways. However, the basics of a lock are relatively standard. Every lock must have a fastening mechanism (i.e., a way to hold together the parts to be "locked"), and a method for engaging and disengaging the fastening mechanism. The fastening mechanism may be physical (such as a deadbolt lock that connects a door to a door jamb) or non-physical (for example, a strong magnetic current in an electromagnetic lock). Similarly, the method for engaging and disengaging the lock can also be physical (such as inserting a key or dialing a combination on a combination lock), magnetic (such as a door that is held in place by the force of two magnets), or electric (such as an electric signal that is generated when the correct keys are pressed on a keypad). Several major parts of typical types of fastening mechanisms are defined below. Different methods for engaging locks are described in more detail in the following sections.

The bolt or latch is the part of a lock that extends into the strike to physically connect two objects. Bolts and latches are typically mounted within a door or on a window and are usually set on a spring so that they can be extended or retracted. When a bolt or latch is engaged, it slides across the open space between the two parts to be fastened, connecting them.

The strike is the part of the lock into which the bolt or latch fits. For example, the strike could be a metal plate with a hollowed out area for the bolt inserted within a door frame.

A hasp is a fastener that consists of at least two parts that fit together or next to each other. An example is the hasp on a metal locker. This hasp consists of a fixed metal ring on the locker frame (usually oriented on the perpendicular) and a moveable metal ring mounted on the door. When the locker door is closed, the ring on the door can be moved over and behind the ring on the locker frame. A combination lock can then be put on the top ring. This lock prevents the bottom ring from moving across the top ring, and thus prevents the locker door from being opened.

A shackle is a physical device by which the parts of a hasp are connected, such as the curved metal piece in a padlock which fits through the rings of the hasp and then inserts into the base of the padlock.

There are many ways to name or describe a lock, including names that describe a lock's mechanical features and names describing the way the lock is used. Locksmiths often refer to locks based on their installation method. For example, a rim lock is a lock that mounts on the surface, or rim, of a door or object. A mortise lock is installed in a hollowed out, or mortised, cavity. In other cases, a generic term can be used to group together different types of locks that have similar features. For example, a "padlock" is a generic term used to describe removable, portable hasp and shackle type locks that consist of a piece of curved metal, both ends of which connect to a base. One of these ends is permanently attached to the base, but the other is attached to the base only when the lock is engaged. When the lock is disengaged, the piece of curved metal can be looped through a hasp to secure an asset, such as a door on a locker or a file cabinet drawer.

Locks can also be defined or described by their functional, or "locking," mechanism. Different types of locking mechanisms include keys, combinations, and electronic or magnetic signals. Keyed locks, which are among the most familiar types of locks, open after a person inserts and turns the correct key. Other locks, such as combination locks, are opened by pressing a series of buttons on a keypad or by turning a dial to the correct sequence of numbers or letters. Some electronic locks are opened by inserting a specially coded "key card." Sophisticated electronic locks open after a computer has identified a feature, such as a fingerprint, of the person desiring access.

Mechanical locks have moving parts that operate without electric current. There are two types of mechanical locks: warded and tumbler.

Warded locks operate through the physical insertion of a key into the lock. These locks have several fixed ridges or obstacles called wards that fit the correct key and block other keys from operating the lock. When a person inserts the correct key, the key fits past the wards and moves a spring inside the lock. The bolt (or shackle) slides into a locked or unlocked position when the spring moves.

Warded locks are easy to pick with a stiff piece of wire or thin strip of metal. Therefore, warded locks are usually used in areas that do not require a high level of security.

Tumbler locks are similar to warded locks except that they have movable metal parts called tumblers that prevent the wrong key from opening the lock. Because tumblers provide more security than wards, most door locks use some type of tumbler arrangement.

There are three types of tumbler locks. These are outlined in Table 1 below.

A combination padlock is a special disk-tumbler lock combined with a padlock. A combination padlock has a movable dial with a series of numbers around it. To open the lock, a person must turn the dial left and right in the correct sequence of numbers.

Electric locks require electric current to operate. Many electric locks include electronic devices with scanners that identify users and computers that process codes. If the correct code or data is received by the scanner, computer, or other input device, an electric current engages or disengages the lock. The specific fastening devices used in electronic locks may be a bolt or an electromagnetic field.

Types of electric locks include:

Card access systems, such as those used in many hotels and office buildings, are the most common electronic lock systems. A person desiring access needs a card or a special "key" to engage/disengage the lock. A device reads the code on the card and sends the information to a computer. If the code matches the one in the computer's memory, the locking mechanism releases and the door opens. Several different card access systems have been developed. One system uses a paperboard or plastic card, on which the code appears as a series of holes or bumps. Another system uses cards or keys that have their code on a microchip or a magnetic strip.

Electronic combination locks are used in many stores and other businesses. To open a typical electronic combination lock, a combination or sequence of numbers must be entered on a numbered keypad. Once the combination is entered, the internal computer compares it with the combination stored in its memory and the door opens if the codes match.

Electromagnetic locks use magnetism rather than bolts to hold a door shut. In these locks, a strike is mounted on the top of the door. A strong electromagnet (a device that acts as a magnet when electric current flows through it) is fastened to the door frame in alignment with the strike. An electric current is put through the system, causing the electromagnet in the door to be attracted to the strike. To disengage the lock, a key is used to stop the flow of current. Doors with electromagnetic locks are often used as emergency exits from buildings. In some electromagnetic systems, the doors automatically unlock when a fire alarm is activated.


Other kinds of electric locks include some types of time locks and delayed-access timers. Time locks are designed to open only at certain times on certain days. They are commonly used on vaults or safes, and the release of the lock can be coordinated with shift changes or working hours. Once the correct combination has been entered, a safe protected by such a timer can only be opened at a pre-set time.

Biometric entry systems are unique forms of electric locks that identify a person by using a computer to compare the unique features of a fingerprint, palm, voice, eye, or signature with the one in its memory. In a fingerprint system, for example, a person who wants to open the lock places his or her finger on a plate. A scanner analyzes the print. If it matches the information in the computer's memory, the lock will disengage. Biometric entry systems are most often used in high-security areas.

Electric locks may be combined with mechanical locks to provide a higher amount of security than electric or mechanical locks alone. For example, electric bolt locks are a type of mechanical lock that provides higher security than magnetic locks. They are available in surface mount or concealed mortise mount and are often used for security applications where electromagnetic locks are not required.

Specifics on different types of locks, including individual information on costs and vendors, are provided in product guides on specific types of locks.