In 1925, 78.26 rpm was chosen as the standard because of the introduction of the electrically powered synchronous turntable motor. This motor ran at 3600 rpm, such that a 46:1 gear ratio would produce 78.26 rpm. In parts of the world that used 50 Hz current, the standard was 77.92 rpm (3,000 rpm with a 77:2 ratio), which was also the speed at which a strobe disc with 77 lines would "stand still" in 50 Hz light (92 lines for 60 Hz). After World War II these records were retroactively known as 78s, <answer>to distinguish them from other newer disc record formats</answer>. Earlier they were just called records, or when there was a need to distinguish them from cylinders, disc records.
Why are these records known as '78s'?
to distinguish them from other newer disc record formats