The Standard Stock title was applied to a variety of Tube stock built between 1923 and 1934, all of which shared the same basic characteristics, but with some detailed differences. This design is sometimes referred to as 1923 Tube Stock, 1923 Stock, or Pre 1938 Stock. Most of the Standard Stock was built to replace the first generation of "Gate Stock" Tube trains or to provide additional trains for extensions built in the 1920s and early 1930s. Standard Stock cars consisted of motor cars, with a driver's cab, behind which was a "switch compartment" occupying approximately one-third of the length of the car, plus trailer cars and "control trailers", with a driving cab but no motor. All were equipped with air operated sliding doors. The guard's door on the earlier trains was a manually operated, inward-opening hinged door. 1922 Stock prototype cars For evaluation purposes, in anticipation of the large number of cars that were to be built over the next several years, six experimental cars were ordered and had been delivered by February 1923. There were five trailers and one control trailer, which were marshalled between French-built "Gate Stock" driving motor cars. A demonstration for the press took place on the Piccadilly line in February 1923, after which the new cars and the French motor cars were moved by road to the Hampstead Line, entering service in August. The French motor cars were part of a batch of 20 which had been rebuilt with air-operated doors, to allow them to work with the 1920 Stock, the first batch of vehicles built with air doors. The trailers and French motor cars formed the inaugural train on the Hampstead Line extension from Golders Green to Hendon when it opened on 19 November 1923. The builders were given a partial specification, which ensured that each car would have 48 seats and two sets of air-operated double doors on each side, providing an opening which was wide. Beyond these basic guidelines, each builder was allowed to build a trailer car to its own design, although externally, they looked very similar. The control trailer was designed by the Underground Electric Railways. The stock became known as the 1922 Stock or Competition Stock and was considered part of the Standard stock fleet. Production stock The earliest Standard Stock was built for use on the Hampstead tube, which was extended from Golders Green to Edgware and