For sports commentaries and anywhere there was a noisy background, it was necessary to speak very closely into the microphone and even then the listener could often hear much more of the background noise than was really wanted.
In 1937 the BBC designed this microphone to overcome this problem. A metal guard ring at the front of the microphone gives a precise speaking distance of 2 1/2 inches when pressed against the top lip.
For this reason these microphones became known as "lip microphones", sounds coming from further away than this distance tend to cancel each other out. The design was improved in 1951 and has since been refined further.