Why are B movies called B movies?

A B movie is a low-budget commercial motion picture conceived neither as an arthouse film nor as pornography. In its original usage, during the so-called Golden Age of Hollywood, the term more precisely identified a film intended for distribution as the less-publicized, bottom half of a double feature. Although the U.S. production of movies intended as second features largely ceased by the end of the 1950s, the term B movie continued to be used in the broader sense it maintains today. In its post–Golden Age usage, there is ambiguity on both sides of the definition: on the one hand, many B movies display a high degree of craft and aesthetic ingenuity; on the other, the primary interest of many inexpensive exploitation films is prurient. In some cases, both are true.

In either usage, most B movies represent a particular genre—the Western was a Golden Age B movie staple, while low-budget science-fiction and horror films became more popular in the 1950s. Early B movies were often part of series in which the star repeatedly played the same character. Almost always shorter than the top-billed films they were paired with, many had running times of 70 minutes or less. The term connoted a general perception that B movies were inferior to the more handsomely budgeted headliners; individual B films were often ignored by critics.