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 also sometimes referred to as 1923 Tube 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, although the guard's door on the earlier trains was a manually op