Skeleton Key Lock Diagram – Old Mortise Lock Parts Named

All 12 parts on the diagram

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The locking side 6

Keyhole The opening the key goes into.
Bit key (skeleton key) The old-style key with a flat bit. Many people call any such key a skeleton key.
Dead bolt The square bolt that the key moves. Nothing but the key slides it out or back.
Lever tumbler A pivoted plate that blocks the bolt. The key lifts it so the bolt can slide.
Wards Fixed ridges inside the lock. Only a key cut to match can turn past them.
Tumbler spring The spring that pushes the lever tumbler back down.

The latch side 3

Latch bolt The bolt with a slanted head. A spring pushes it out and the knob draws it in.
Hub The round piece the knob spindle passes through. Turning the knob turns it and draws the latch bolt in.
Latch spring The spring that pushes the latch bolt back out.

Case, cover and front 3

Case The box that holds the bolts and the working parts.
Cover (cap) The removable lid of the case. Lock makers also call it the cap.
Front (faceplate) The plate on the edge of the door. The ends of both bolts pass through it.

How this list was compiled: Every name read on 2026-10-03 and kept only where two independent sources give it. (1) Henry R. Towne, Locks and Builders' Hardware: A Hand Book for Architects (1904, public domain), the glossary and the lock primer, https://archive.org/details/locksandbuilder00towngoog : bit and bit-key lock, key-hole, ward, dead bolt, latch bolt, tumbler and lever tumbler, hub, case, cap (also called cover), front (the face plate of a mortise lock), and a skeleton key as one whose bit is cut away to pass the wards. (2) Wikipedia, text only: Warded lock, Lever tumbler lock, Mortise lock and Skeleton key, https://en.wikipedia.org/wiki/Lever_tumbler_lock : wards, levers pushed to rest by a spring, the bolt, the case, the keyhole, a sprung latch beside a locking bolt, the bit of the key. (3) Read by full-text search at archive.org: a correspondence school's text on builders' hardware, https://archive.org/details/bwb_C0-BNX-744 (the latch bolt worked by the spindle through the square hole or hub; the dead bolt worked by a key); a lock maker's catalog of 1937, whose parts list of a mortise lock names front, case, latch bolt, dead bolt, knob hub, latch spring, tumbler and tumbler spring, https://archive.org/details/sager-catalog-1937 ; a glossary in an architecture magazine of January 1936 (front: the face plate of a mortise lock), https://archive.org/details/usmodernist-PP-1936-01 ; and a 1983 locksmithing manual whose figure of a lever tumbler lock names the cover, https://archive.org/details/completebookoflo00rope . The drawing is our own. Licence: own work

The drawing: Drawn for this site on 2026-10-03 from plain geometry. Own original drawing; nothing traced or copied. Not to scale. Licence: own work

How it works

A skeleton key lock is an old mortise lock worked by a bit key. Inside the case are two bolts: a latch bolt that the knob draws in, and a dead bolt that only the key moves.

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The locking side

The key goes in at the keyhole and turns past the wards. Its bit lifts the lever tumbler, which frees the dead bolt, and then pushes the bolt out or back. The tumbler spring drops the lever again.

The latch side

The knob spindle passes through the square hole of the hub. Turning the knob turns the hub and draws the latch bolt in; the latch spring pushes it back out. The front is the plate on the door edge that both bolts pass through.

To tell the two bolts apart: the latch bolt has a slanted end, the dead bolt a square one.

Questions and answers

What are the parts of a skeleton key lock?

A case with a cover and a front; a dead bolt, a lever tumbler with its spring, and wards beside the keyhole; and a latch bolt with its spring, worked by the hub that the knob spindle turns.

How does a skeleton key lock work?

The key turns past the wards and its bit lifts the lever tumbler. With the lever lifted, the dead bolt is free, and the bit slides it out to lock or back to unlock.

Why is it called a skeleton key?

The name first meant a key cut down to its bare essentials, used as a master key for a set of warded locks. Today people use it for any old key with a round shank and a flat bit.

What is a mortise lock?

A lock set into a pocket, the mortise, cut in the edge of the door, so that only its front shows. Old bit-key locks were made this way or screwed to the face of the door as rim locks.