It carries the sash and holds it at any position by friction.

Hinge and brake are the same component.

Articulated arms set the opening geometry.

Controlled friction in the pivots resists movement.

The window stays where it was left without a separate catch.

That combination is what makes the part specific rather than generic.

By sash weight and size together, from the manufacturer's chart.

Both figures are needed rather than one.

Glass weight is what people consistently underestimate.

Double and triple glazing are considerably heavier than they look.

A stay chosen for the frame rather than the glazed sash is under-specified.

Charts exist because this cannot be judged by eye.

It works when new and fails gradually over months.

The sash begins to sag or drop at the leading edge.

It binds against the frame or will not close cleanly.

By the time the symptom is obvious the stay is usually distorted.

The frame may have been damaged as well.

That delayed failure is why specifying correctly at the outset matters.

Generally yes, so the sash is carried evenly.

A new stay alongside a worn one loads them unequally.

It also makes even adjustment impossible.

Both aged in the same conditions, so one failing suggests the other is close.

Handing and fixing pattern must match the existing window.

Photographing the fixing positions before removal saves measuring afterwards.

Within a limited range on most patterns, and it matters in both directions.

Too little and the window moves in the wind.

Too much and it is hard to open and stresses the fixings.

Adjustment is usually a screw on the stay arm.

It should be set with the sash fully glazed.

A stay that will not hold at any setting has usually worn or corroded past recovery.

A seized stay is almost always a corroding one.

It is a steel component in a wet location near the outside face.

Corrosion binds the arms and the pivots.

Debris in the track contributes as well.

Cleaning and lubricating helps early and not once pitting has begun.

Stainless grades are specified for coastal and exposed work as a matter of course.

Frequently not - an ordinary stay leaves the sash across the opening.

The clear opening is therefore much smaller than the window.

Escape routes generally require a stated clear dimension.

Egress hinges swing the sash clear to provide it.

Which windows must serve as escape routes is set locally.

Where a window is designated, an ordinary friction stay is usually not sufficient.