An interconnected lock is actually two locks that are connected by an assembly that retracts both the deadbolt and the latch simultaneously when the inside handle is turned. This is done to fulfill the life safety requirement under NFPA 101 that egress should be accomplished by one motion with no prior knowledge necessary, and at the same time provide the user with the security of a deadbolt. The same function could be provided by an entry function mortise lock, but interconnected locks are cheaper, since they are usually cobbled together out of (usually) a grade 2 cylindrical lock and a tubular deadbolt.
The history of the interconnected lock is a twisted, strange story of different companies reinventing the wheel with different distances between the centerlines, connected or separate latch/bolt assemblies with correspondingly different strike preps, and radically different hole patterns on both interior and exterior door surfaces. The end result has been many, many doors and frames prepped for locks that are now irreplaceable.
Today, preps are much more standardized.
These are some of the interconnected locks available today and the measurements of their preps:
- Falcon H Series – 4 inches CTC, 1-3/4 inch hold above, 2-1/8 inch hole below
- Schlage H Series – 4 inches CTC, 1-1/2 inch hole above, 2-1/8 inch hole below
- Schlage S200 Series – 4 inches CTC, 1-1/2 inch hole above, 2-1/8 inch hole below
- Schlage CS200 Series – 4 inches CTC, 2-1/8 inch holes above and below
- Sargent 75 Series – 4 inches CTC, 2-1/8 inch holes above and below
- Yale 4800LN series – 4 inches CTC, 2-1/8 inch holes above and below
Replacing any of the locks above with any of the others would not present an enormous problem.