Double-cylinder deadbolt, Grade 1

A door with a glass panel or a sidelight beside the lock has a thumbturn that can be reached through it. A double cylinder is keyed on both faces, so breaking the glass gets an intruder no further.

The trade-off is real and we will state it: in a fire, a double cylinder needs its key to open from inside. Keep the key on a hook within reach of the door but not within reach of the glass, and do not fit one to the only exit from a bedroom floor. Some jurisdictions restrict them on required egress doors — check yours before ordering.

Fitting and care. Decide where the inside key lives before you fit it, and make that a hook that is fixed, within reach of the door, and out of arm’s reach of any glass. Tell everyone in the house where it is. Do not fit this to a door that is the only exit from a sleeping floor. Check your local code first — several jurisdictions restrict double cylinders on required egress doors, and a lock that fails an inspection has to come off. Lubricate the cylinders annually with graphite or PTFE, both sides.