What Is a Double Cylinder Lock
A double cylinder lock is a door lock that requires a key on both the exterior and the interior to lock or unlock. Unlike a single-cylinder lock-where you use a key outside and a thumbturn inside-a double cylinder replaces the inside thumbturn with a keyed cylinder. The defining behavior is simple: no key, no operation from either side.
Because it can prevent quick egress, the double cylinder format sits at the intersection of security, safety, and building code. Used in the right context it mitigates certain break-in risks; used in the wrong context it can violate egress rules and create life-safety hazards. This article explains how the format works, where it's appropriate, what the alternatives are, and how to spec or maintain one responsibly.
Core Idea and How It Works
In most lock families, "double cylinder" means two independent key plugs driving the same locking mechanism:
Deadbolts (residential/commercial): A keyed plug on the outside and inside operates the bolt. There is no thumbturn.
Euro profile cylinders (DIN/EN): A double euro is key/key; turning either key moves the cam in the mortise case.
Mortise/rim cases (commercial/storefront): A keyed cylinder is fitted on both sides of the door to throw/retract the bolt or latch.
Mechanically, operation is identical to a single cylinder-only the interface changes (key vs thumbturn). Electrically/electromechanically controlled versions also exist; they keep the "key on both sides" concept but may include readers or electrified trims.
Why People Choose Double Cylinders
Glazing near the lock: If a door has glass sidelites or a window within arm's reach of the thumbturn, an intruder could smash glass and twist the thumbturn. Replacing the thumbturn with an interior key cylinder eliminates that easy attack.
Public-facing or at-risk locations: Shops with breakable storefronts, short setback entries, or doors that open to alleys sometimes choose double cylinders to reduce opportunistic break-ins.
Key control policies: Some facilities prefer that all locking actions require a key so staff can manage who can secure or unsecure a space.
Serious Caveat: Egress and Code
In many jurisdictions-especially in the U.S.-egress doors must be openable from the inside without a key, special knowledge, or effort when the building is occupied. That typically prohibits double-cylinder deadbolts on residential exit doors and most commercial exit paths. Enforcement varies by country, state/province, and occupancy type, so you must check your local building and fire codes (and your insurance conditions) before specifying or installing a double cylinder.
Key risks to consider:
Life safety: In an emergency (fire, smoke), a missing or misplaced interior key can trap occupants.
Compliance exposure: Non-compliant hardware can trigger citations, denied inspections, or liability after an incident.
Operational friction: Day-to-day users may forget to remove a key or may re-lock a space incorrectly.
Practical approach: If you're considering a double cylinder for a primary egress door, talk with your authority having jurisdiction (AHJ) or a qualified door hardware consultant. In many cases, a safer alternative delivers similar security with compliant egress (see below).
Variants and Terminology by Lock Family
Deadbolt (ANSI/BHMA):
Single cylinder: key outside, thumbturn inside.
Double cylinder: key both sides.
Classroom/entry variations: keys control the bolt from one or both sides, but inside egress may be free via the lever set; function numbers vary by manufacturer-confirm the exact function sheet.
Euro profile cylinders:
Double (key/key): keys both sides.
Thumbturn (key/turn): key outside, turn inside.
Half (single-sided): key on one side only (e.g., garage door).
Options like emergency function allow an outside key to work even if a key is left in the inside cylinder (this is not the same as a double cylinder but is often paired with double-euro systems for convenience on non-egress doors).
Mortise/rim systems:
Common on aluminum storefronts and commercial doors; you can fit keyed cylinders both sides of the case. Egress rules still apply to the overall door function.
Pros and Cons
Advantages
Mitigates "reach-through" attacks: No interior thumbturn to twist after breaking glass.
Key-controlled locking on both sides: Useful for spaces that must not be secured/unsecured casually by anyone inside.
Consistent key policy: One credential governs all operations.
Disadvantages
Egress hazard: Occupants need a key to exit. This is the primary concern and often a code violation.
Operational complexity: Keys can be misplaced, forgotten, or locked inside.
Emergency response: First responders may be delayed if a double cylinder is installed on a required exit.
Safer, Often-Preferred Alternatives
If your aim is to stop "reach-through the glass and unlock," these options preserve key-free egress while improving security:
Single cylinder + security glazing or guards
Use laminated or tempered laminated glass, security film, or window guards to make smashing/reaching impractical.
Single cylinder + high-security thumbturn
Some thumbturns are shrouded, small-throw, or recessed to reduce tool leverage through broken glass.
Exit device (panic hardware) with exterior cylinder control
Inside: always-free egress via push bar. Outside: key (or reader) controls entry. This is the gold standard for many commercial doors on egress paths.
Classroom/office functions on lever locks
Inside lever remains free for egress; a key on either side changes the outside status. You get controlled locking without trapping occupants.
Euro cylinder with "emergency function"
Allows an exterior key to operate even if a key is inserted inside (helps avoid lockouts). Pair with security glazing rather than going full double-key on egress doors.
Where Double Cylinders Still Make Sense
Utility/maintenance rooms not on a required egress path.
Roll-down gates or grilles where egress isn't an issue (rules vary).
Secondary perimeter doors used only by trained staff with clear procedures (and only if code allows).
Cabinets, gates, and interior partitions where emergency egress isn't applicable.
Even in these scenarios, post clear procedures and ensure keys are immediately available to authorized personnel.
Selecting a Double Cylinder (If Allowed)
1) Confirm code status
Get a thumbs-up from your AHJ for the specific door and occupancy.
2) Choose the right format
Residential deadbolt? Match backset (2-3/8" or 2-3/4") and door thickness.
Euro profile? Specify outside/inside lengths from the fixing screw center (e.g., 40/45 mm).
Mortise/rim? Confirm cam type and case compatibility.
3) Specify keyway and key control
Use a keyway that fits your system (e.g., Schlage C, Sargent LA, Best SFIC). For facilities, consider restricted (patented) keyways to prevent unauthorized duplication.
4) Minimize exterior projection
Pair with hardened escutcheons; on Euro cylinders, keep projection ≤2–3 mm to reduce snapping risk.
5) Consider security features
Anti-drill plates, anti-snap (Euro), clutching, pick resistance, and hardened inserts.
6) Plan for operations
Define who holds inside keys, where spares live, and how emergency egress is protected in practice.
Installation and Use Tips
Door prep and alignment: A misaligned strike creates extra torque and shortens cylinder life.
Through-bolting where possible: Strengthens the assembly on commercial hardware.
Key discipline: If policy requires an interior key to remain nearby, mount a break-glass key box or high hook out of reach of windows yet obvious to occupants (subject to local rules).
Labeling: In facilities, label interior key positions or provide signage so staff know how to secure/unsecure without trapping occupants.
Maintenance
Quarterly: Lightly blow out the keyway; apply dry lubricant (graphite/PTFE). Avoid oily sprays.
Annually: Verify screws and escutcheons are tight; ensure door closes and latches without lifting or forcing.
Key health: Replace worn keys with code-cut copies; duplicating from a worn key accelerates cylinder wear.
Frequently Asked Questions
Is a double cylinder lock more secure than a single cylinder?
Against reach-through attacks, yes. Against other forced-entry methods, overall door construction, glazing, and frame strength matter more than the cylinder type alone.
Is it legal on my front door?
In many places, no-egress doors must open from the inside without a key. Always check local code and your insurance terms.
What about leaving a key in the inside cylinder?
Some users hang a key near the door for quick exit. That can defeat the security purpose and may still be non-compliant; consult your AHJ.
Can I get the same key for both sides?
Yes-the two cylinders are usually keyed alike. Facilities often master-key the lock so managers and responders retain access.
What's the best alternative if I'm worried about glass?
Single cylinder + laminated/security glazing or panic hardware with controlled exterior entry. These options raise security while keeping key-free egress.
