Key and Thumbturn Euro Cylinders: Which Side the Turn Goes, and How to Split the Length

On a key-and-thumbturn euro cylinder, the thumbturn goes on the inside, the secure side, every time. The key half faces whoever should need a key: the street, the corridor, the common stair. The turn faces the room the occupant locks from and leaves from. A thumbturn is, in effect, a key that never leaves the door, so it can only sit on the side where anyone who touches it is already entitled to be there.

The length is split the way any euro cylinder is split: two halves, each measured from the cam center line (where the fixing screw passes through) to the end of the body. The difference with a thumbturn is that the split has a fixed direction. A double-key cylinder ordered 30/40 can be turned round and fitted as 40/30. A key-and-turn cylinder ordered 30/40 cannot, because turning it round puts the turn on the outside. So the order has to say which half carries the key and which carries the turn, and the turn half is measured to the end of the body, never to the tip of the turn.

Why the turn always goes inside

A thumbturn operates the cam with no key and no skill. On the outside face it would make the lock decorative. On the inside face it does the job it was chosen for: the occupant can lock behind them at night and get out in a hurry without looking for a key, which is the main reason to choose a key-and-turn cylinder over a double-key one in the first place. The inside face is also the one nobody attacks from, so it is the face where a little extra projection costs nothing.

Door handing does not change the rule. A euro cylinder is not left- or right-handed; it is outside or inside. The decision that does need making first, on doors between two rooms or doors that open outward, is which side is the secure one. Between two offices it is the room whose occupant locks the door. On an apartment or house entrance it is the dwelling. Write that down before anyone measures, because the measurement depends on it.

How the split is measured, and why the turn half is different

The key half is measured from the cam center line to the outer end of the body, the face with the keyway. Its target is set by security: it should finish flush with the escutcheon or security rose, or only just proud, because a key half that stands out gives a tool something to grip. That rule, and the arithmetic from door thickness, are covered in our notes on cylinder length and are not repeated here.

The turn half is measured from the same cam center line to the end of the body where the turn's stem comes out. The knob and its stem are not part of the length: do not add them, and do not measure an old cylinder to the tip of its turn. If the old cylinder is out of the door, lay a rule from the fixing screw hole to the body face on each side, and stop at the metal of the body, not at the part you hold.

What makes the turn half different is its tolerance. It does not need to be flush for security, so it is the half that can absorb surplus. When the door does not land exactly on an available split, put the extra millimeters on the turn side and keep the key side tight. The limits on the inside are practical: the turn must clear the handle or the plate so it can be gripped, and a turn half that stands far out looks unfinished and can be knocked.

Our 70 SNKT-2 shows how a published split can read two ways. Its record gives 30/10/30mm: two 30mm body sections either side of a 10mm cam section, 70mm overall. Measured the usual way, to the cam center, the same cylinder is 35/35. Because it is symmetric, it has no wrong way round for length; only the turn's side matters, and that side is inside. Our other key-and-turn cylinders are listed at 60, 65, 70, 80 and 90mm overall, and their records do not publish a split, so we do not fill one in. One caution about our own codes: the photographs of models such as 70 SNKT-1, 70 SNKT-2 and 80 SNKT show a key at one end and a thumbturn at the other, but we have not published KT as a formal code definition, and suppliers do not all use those letters the same way. Write the function in words.

How to order the split

One line on the order removes almost every error: the function in words, the overall length, the key half labeled, the turn half labeled, and the convention. For example: euro profile, key and thumbturn, 70mm overall, key side 30 / turn side 40, both measured to cam center, satin nickel, turn inside. Some suppliers mark the turn half with a T, as in 30/40T; the habit varies, so let the words carry the meaning and treat the letter as a check.

Where a thumbturn should not be used

Next to glass that can be reached through. A glazed panel in the door, a sidelight beside the lock, or a vision panel within arm's reach of the cylinder lets somebody break the glass and turn the lock from inside. A letter plate close to the lock is a smaller version of the same problem, because tools pushed through it can work a turn. The options are laminated glazing, moving the lock away from the glass, a thumbturn that can itself be locked with a key, or a double-key cylinder, and the last one only where the next point allows it.

On escape routes where a key-free exit is required. Building rules in many countries require a door on a means of escape to open from the inside without a key, tool or special knowledge. A double-key cylinder fails that; a thumbturn or a lever-operated latch is the usual answer. Some rules go further and ask for a single action to release the door, in which case a deadbolt turn in addition to a latch handle can count as a second action, and US accessibility rules ask that operating parts work without tight grasping, pinching or twisting of the wrist, which a small turn may not meet. These rules differ by country and building type, and we describe them as background rather than as a ruling: the authority having jurisdiction, or the fire engineer, decides for a given door.

Where a key-free exit is forbidden. A smaller group of doors is deliberately designed so that people cannot leave freely: secure areas in some healthcare settings, custodial buildings, and doors that are on no escape route and must stay locked in both directions. Those arrangements are set by the responsible authority, usually with a controlled release, and a thumbturn defeats them. If a door is neither an escape door nor a controlled one, the choice is a judgment between exit and security, and some insurers have a view on doors with glass near the lock; read the policy wording rather than assuming what it says.

Double key or key and thumbturn

The usual argument for a double-key cylinder is glass. The usual argument against it is the key that is not in the lock when it is needed. On a dwelling entrance with no glass in reach, a key-and-turn cylinder answers both. On a door where glass is in reach and the door is also an exit, neither cylinder is the whole answer, and the glazing is the part to change. In our range the two sit side by side at the same length and finish: 70 SNDK is listed with double key function, and 70 SNKT-2 is photographed with a key and a turn.

Emergency-function and clutch cylinders

Inside a double-key euro cylinder, a clutch decides which plug drives the cam. In many designs a key left in one face holds the clutch, and a key from the other face then turns without effect. An emergency-function cylinder, also sold as a clutch or double-function cylinder, is built so that a key on one face still operates the cam with a key in the other. It matters where a double cylinder is chosen but someone may need to get in from outside while a key is left on the inside. Which face takes priority, and whether both do, varies between designs, so ask rather than assume.

A key-and-turn cylinder does not have the left-key problem in the same form, because there is no inside key; the turn stays on its own side of the clutch. The records for our cylinders do not publish whether an emergency function is fitted. If you need one, say so in words on the inquiry, and we will answer for the specific model rather than for the range.

With any inquiry, send the door thickness, the escutcheon or rose on each face, which face is inside, the cylinder function in words, and whether the door has glass near the lock or sits on an escape route. Those facts settle the split and the function. If the cylinder comes inside a handle set, such as our 9088 SS stainless lever set, whose record states a euro cylinder with key outside and thumbturn inside, ask for the cylinder length as well: that record does not publish it.