The indicator is mechanically tied to the bolt. Turning the thumbturn throws the bolt into the keeper and rotates a coloured disc behind an aperture in the escutcheon, so the colour outside is driven by the bolt position rather than by a separate switch.

Because the two move together the display cannot disagree with the door. A bolt only partly thrown shows a partial colour, which is a genuine signal that the door is not properly secured.

Colour conventions vary by market, so a green and red pair, a vacant and engaged legend, or a plain colour block may all appear. In a building used by an international public, colour rather than wording carries the message.

Emergency-release patterns are designed for exactly that. The outer face carries a coin slot or a squared spindle that overrides the thumbturn, so a member of staff can withdraw the bolt from the corridor side without tools or force.

This is the reason cubicle doors are often hung to swing outward, or hung on lift-off hinges: even with the bolt released, a collapsed occupant can block an inward-opening leaf.

Not every latch offers it, and a plain slide bolt offers none. Where the washroom serves the public, a school or a care setting, override capability is usually treated as a requirement rather than an upgrade.

The keeper is the plate and housing on the pilaster that receives the bolt. It takes the impact every time the door is closed hard and the friction every time the bolt is thrown, so it accumulates more wear per cycle than the latch body.

As the keeper mouth widens the door starts to rattle in the closed position, and the bolt engages less deeply. Left alone, the bolt eventually rides over the lip instead of dropping into it.

Because it wears predictably, the keeper is the part most often replaced on its own, and matching the throw depth of the existing bolt matters more than matching the plate outline.

The stop sets where the closed door comes to rest, and the latch can only engage correctly if that position is right. A stop set too deep lets the door pass the keeper and the bolt jams against the plate face; set too shallow, the bolt cannot reach.

It also absorbs the closing impact. A rubber or nylon stop takes the energy that would otherwise go into the keeper and the door edge, which is why replacing a missing stop often cures a latch that has started to bind.

On outward-opening doors the stop is on the frame side rather than the pilaster, and it is the component that stops the leaf swinging into the pilaster face.

They use a different geometry rather than a different mechanism. Sloped and domed profiles, closed escutcheons and the removal of any projecting lever or gap mean nothing can take a cord or a strap.

The override is usually strengthened at the same time, so staff access is quicker and more certain than a coin slot allows, and the indicator may be enlarged so occupancy is readable at a distance.

These patterns are specified for mental health, custodial and some education settings, and they are not interchangeable with standard latches: the fixing centres, the door edge preparation and the keeper all differ.

Almost always movement somewhere other than the latch. A pilaster settling on its shoe, a headrail bracket working loose or a floor screed that has moved will drop the door a few millimetres, and the bolt then meets the keeper lip.

Hinge wear does the same thing more gradually. As the pivot bushes wear, the door hangs slightly lower and further out, which shows first as a bolt that needs the door pulled in to engage.

Adjusting the latch to suit a sagging door hides the cause and moves the wear somewhere else. Levelling the pilaster or renewing the hinge restores the geometry the latch was drilled for.

By the barrel or spindle length. The latch body sits in or on the door edge and its throw is measured from that face, so the same visible escutcheon can front several internal lengths.

Compact laminate cubicle doors are commonly 13 mm, 20 mm or 25 mm, and solid grade laminate can be thicker again. Fitting a latch cut for a thin door to a thick one leaves the bolt too short to reach the keeper.

Where doors are being replaced but pilasters retained, the door thickness is the variable that changes, so latch throw is checked against the existing keeper position rather than assumed from the old latch.