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A latch is a sliding mechanism that prevents a door from opening. It has evolved from simple crossbars to modern doorknob mechanisms. The latch can be part of a deadbolt and is still commonly used in many places. Two-bolt systems are common in higher security doors.
A latch is the part of a door that prevents the door from opening. To be a latch, the element must slide into place. This separates the latch from chain locks or hooks, where the locking mechanism is simply placed in a holder. The actual deadbolt can be in any number of forms, from a standard doorknob catch to a deadbolt. Latches are one of the oldest forms of door locking technology and are still very common.
Bolts were used as a way to keep doors closed as long as people used the doors. The simplest and one of the earliest forms of bolt is called a crossbar. They were simply long bars that went into the door brackets. When in place, the door does not open. This method not only keeps the door closed but also locks it. This form of latch is still common in many places, including many public restrooms.
As locking technology has improved, the latch has improved as well. As doors began to internalize their fastening structures, the latch became part of the internal doorknob mechanism. Modern bolts work in the same way as their historical counterparts: the only difference is the method used to slide them. In the old days, human strength moved the latch in and out of position on the door. With modern doorknobs, mechanical action moves the bolt.
When a typical doorknob turns, the mechanical action pulls the bolt out of the door jam. The inside of the knob will typically contain a spring that pushes the latch into place if the door knob is released. When the door is closed, the knob does not turn and the bolt does not disengage. If the door is open and locked, the spring will allow the latch to retract, even if the knob doesn’t turn; this allows a person to close a locked door but not to reopen it.
When the door has two latches that jam the door jamming into separate openings, typically one locks and the other doesn’t. What gets stuck is the latch and what doesn’t is the latch. This configuration is common on higher security doors, such as exterior doors, or with deadbolt systems. In many cases, a two bolt system will not allow a person to close a locked door like a normal doorknob since the locking bolt will not move at all when locked.
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