16mm

 was a standard film format, so named because of its 16-millimeter width.

It was the usual format of choice for location filming until the programme switched exclusively to 1" Type C videotape during season 23. Consequently, it was the film stock used when a strike forced the whole of Spearhead from Space to go on location.  Spearhead is thus the only Doctor Who story entirely filmed on 16mm, though many others have large sections recorded on the format.

Additionally, it was the preferred format for telerecording. Almost every 1960s and early 1970s episode which survives as a telerecording does so in 16mm.