Mercury Living Presence was one of the early stereo techniques of the 1950s, used for classical music recordings on the Mercury label. If you imagine trying to figure out how to make a stereo recording when there was no-one around to tell you how to do it, you might work out that one microphone pointing left, another pointing center and a third pointing right might be the way to do it.
Record each to its own track on 35mm magnetic film, as used in cinema audio, and there you have it! Nominally omnidirectional microphones were used, but of course the early omni mics did become directional at higher frequencies. Later recordings were made to two-track stereo. These recordings stand up remarkable well today. They may have a little noise and distortion, but the sound is wonderfully clear and alive.