What it is
A second, complete glazed line installed inside the existing window, with its own frame, its own seals and its own opening arrangement. The original window is untouched and stays fully operable behind it.
This is different from an insert bonded into the existing frame — see acoustic window inserts for the lighter-weight version of the same idea.
Why the wide gap wins on noise
Acoustic performance between two panes depends strongly on how far apart they are. A sealed double-glazed unit has 12 to 20 mm between panes, limited by what the edge seal can span.
A secondary system has no such constraint. A 100 mm or wider gap can be left between the existing window and the new inner line, and that distance is considerably more effective against the low-frequency end of traffic noise — the part you feel as much as hear, and the part that a sealed unit struggles with.
The second factor is decoupling. Two independent frames do not transmit vibration to each other the way two panes joined by a spacer bar do.
For rooms where this is the whole objective, see soundproof windows.
Where it is the only option
Heritage and protected elevations. Nothing visible from outside changes, and the work is reversible. That is usually what makes approval possible at all — see theatres for the type of building where this comes up.
Society-controlled buildings. Many Mumbai societies require a consistent external appearance. Secondary glazing does not alter it.
Rented flats. Reversible, so it does not require a landlord to agree to permanent alteration.
Operating hotels. Room-by-room installation with no external access and no scaffolding — see hotels.
Condensation, which needs designing
The one technical pitfall. If humid indoor air can reach the cold outer pane, it will condense there.
The principle is that the inner line should be the tighter seal, so indoor air does not readily reach the cavity, and the cavity should have a small controlled vent to outside so any moisture dries outward rather than inward. Getting this backwards produces a permanently misted original window and is the most common complaint about badly installed secondary glazing.
Operation
Secondary units are made to slide, hinge or lift out, so the original window behind still opens. Which arrangement suits depends on the existing window — a casement behind a fixed secondary pane is unusable, so the secondary line has to match how the original works.
What it does not do
It does not fix a rotten frame, and it does not improve the appearance of a failing window — it puts a new window in front of an old one. Where the original is past saving, glass replacement or full replacement is the honest answer.