Three dimensions, none of which are about appearance.

Spindle size has to match what the gearbox accepts.

Spindle length has to suit the sash depth plus any packing.

Fixing centres have to align with the existing holes.

Getting any one wrong means the handle does not work.

Measuring all three before ordering avoids the usual disappointment.

Because it depends on the sash depth and anything packing the handle out.

Too short and the spindle does not reach into the gearbox reliably.

Too long and the handle stands off the sash.

That looks wrong and eventually damages the mechanism as it is forced.

Packing plates change the requirement entirely.

Measuring the existing spindle before removing it is the simplest approach.

Many patterns are, and conventions differ between manufacturers.

A handle may be made specifically left or right.

How handing is defined varies, so assuming is risky.

Checking the manufacturer's own definition avoids a return.

Some patterns are reversible or universal.

Those sidestep the question entirely and suit mixed installations.

Yes, provided the spindle and fixing centres both match.

The gearbox is a separate component from the handle.

A matching handle drives the existing gear unchanged.

A different centre distance means new holes in the sash.

On uPVC and aluminium that is not always practical.

Where the gearbox itself has failed, that is a different and larger job.

Yes - it decides how the handle survives its environment.

Coastal and exposed locations attack many finishes quickly.

Salt spray performance is published for exterior hardware.

Internal handles have a considerably easier life.

Matching finish across a building is usually a design requirement.

Long-term availability of the same finish matters on a large project.

The gearbox or the locking gear is usually the cause rather than the handle.

Debris and corrosion bind the mechanism internally.

A dropped or misaligned sash makes the gear work against the frame.

Lubrication helps early and not once corrosion has started.

Forcing a stiff handle breaks the spindle or the gearbox.

Adjusting the hinges or stays frequently resolves what looks like a handle fault.

It depends on the window's position and on what the insurer requires.

Ground-floor and accessible windows are frequently specified as lockable.

A locked window is also harder to escape through.

Where the window is a designated escape route that matters a great deal.

Egress requirements differ between markets and take precedence.

Keeping the key nearby rather than in the lock is the usual compromise.