Queen Alexandra's State Coach is one of several state carriages maintained at the Royal Mews, Buckingham Palace.
It was built around the year 1865, initially as a plain "town coach".
Some 30 years later it was glazed and converted into a State Coach for the use of the Princess of Wales (later Queen) Alexandra.
(In this instance it is always accompanied by The King's Bargemaster and Watermen acting as footmen, a reminder of the days when the Crown Jewels were invariably conveyed from the Tower of London by river for State occasions.)
In transit, like the monarch himself, the crown and insignia are entitled to a Household Cavalry escort and receive a royal salute.