A night latch is a type of auxiliary lock often used in conjunction with a regular latchset on an exterior door. These locks have largely been replaced by deadbolts in the U.S., but are still common in Europe and many other parts of the world. Traditionally the night latch was designed so that it could not be unlocked from the outside. Because it could only be locked when the occupants were home, it eventually developed the name “night latch.” Modern units often have a keyed cylinder that allows you to open the lock from outside as well as a thumb turn to lock the unit from inside the home.
Choose the location for your night latch. If a template was supplied with your lock, use it to determine the lock's placement. If not, place the lock on the edge of the door at 46 to 48 inches above the floor. This is a standard location for most deadbolts and auxiliary locksets.
Place the lock body against the interior side of the door at your chosen elevation. Use a pencil to mark the screw holes for the lock. Place the strike on the edge of the frame so that it is aligned with the lock, and use your pencil to mark the holes for the strike.
Pre-drill the screw holes for both the lock and strike. Using the lock template as a guide, drill a hole for the cylinder with your hole saw. If no template is provided, measure the centerline of the cylinder in relation to the rest of the lock. Mark this spot on the door so it is in the correct location based on the screw holes you've drilled. Measure the diameter of the cylinder, and bore a hole of the same diameter to accommodate the cylinder.
Slide the cylinder ring around the body of the cylinder, and insert it through the exterior side of the door. The keyhole should be right-side up (so that the jagged teeth of the key would face up if the key was inserted).
Place the night latch on the interior side of the door, and line up the tailpiece on the cylinder so it fits into the night latch. Hold the two units together, and add screws to the holes you pre-drilled in step 3.
Mount the strike on the door so that it is aligned with the night latch. Fasten it to the door using screws. Test the lock to make sure it works from both the interior and exterior sides of the door before proceeding.