In the days of mono, the needle only moved side to side while tracing through the groove. Later, the format was designed so that the stereo information could be read from the up and down movement of the needle. This was done to ensure that older, mono only record players could still play newer stereo records.
This is the first I’ve heard this, but it makes so much sense. It’s very clever. Basically, the audio on vinyl is encoded mid/side. The mid (mono) audio is the side-to-side movement (lateral), and the side (stereo) audio is the up-down movement (vertical). That explains why the needle might jump out of the groove if there’s too much strong bass content in the side channel. It also explains why the Fairchild 670 compressor’s mid/side mode is called lateral/vertical.
The article also has a good explanation of why audio fidelity is better on the outside of a record (at the start).