A cam on one sash rotates into a keeper on the other.

The cam's shape draws the two sashes tight as it turns.

That locks them and closes the gap between them.

Both happen in the same movement.

It is fitted at the meeting rail where the sashes overlap.

Pulling the sashes together is half of what it is for.

Noticeably - sliding sash windows are draughty at the meeting rail.

A lock that pulls the sashes together closes that gap.

Weather performance improves as much as security.

A worn or badly positioned lock no longer draws them tight.

The window then rattles and draughts even when locked.

Repositioning the keeper frequently restores it.

Limited - it is surface mounted and held by screws into the meeting rail.

Its resistance is the strength of those screws.

Against someone breaking glass and reaching in, an unlocked one offers nothing.

Key-locking versions exist for that reason.

Insurers frequently require them on accessible windows.

The glazing remains the weakest element regardless of the lock.

Because sash windows move and the alignment has drifted.

Timber swells and shrinks with the seasons.

Cords stretch and sashes settle.

A lock engaging perfectly in summer may not in winter.

Adjusting or repositioning the keeper is the usual remedy.

Replacing the lock rarely helps if the alignment is what changed.

That needs a stop or restrictor rather than the sweep lock.

A sweep lock is either engaged or not.

Sash stops allow limited opening while remaining secure.

Restrictors serve child safety in the same way.

Where the window is an escape route, local rules govern.

Whether a restricting device may be fitted at all is a regulatory question.

On period work it frequently matters as much as function.

The lock is visible on the meeting rail.

It commonly has to suit the building rather than the catalogue.

Brass, bronze, chrome and painted finishes are all available.

Conservation work may specify a particular pattern.

Corrosion resistance still matters where the window is exposed.

Straightforward, provided the sashes align correctly first.

The cam and keeper positions are marked from each other.

Fitting to a misaligned window locks in the misalignment.

Screws into sound timber matter more than the lock itself.

Old screw holes may need filling before refixing.

Getting the sashes meeting properly before fitting is what makes the lock work.