Two channels in one moving groove
A stereo record does not give each channel its own track. Instead, the cutting stylus that makes the master groove moves in response to both audio signals at once. The playback stylus follows those tiny changes, and its motion is converted into an electrical signal by the cartridge—the small assembly at the end of the tonearm that holds the stylus and senses its movement.
The 45/45 arrangement
The groove’s two sloping walls are set at roughly 45 degrees to the record surface. Each wall carries one channel: the left channel’s variations are cut into one wall, and the right channel’s into the other. As the stylus rides through the groove, it can move both side to side and up and down. Those combined movements let it follow the two wall patterns simultaneously.
This arrangement also explains why stereo playback depends on reading the stylus’s motion in more than one direction. A purely side-to-side movement represents sound shared by the channels; movement up and down reflects differences between them. The cartridge separates those motions back into left and right electrical signals, which an amplifier sends to the corresponding speakers.
A compact physical encoding
The clever part is that stereo is not stored as two parallel grooves, which would require more space and make tracking harder. The two channels are interwoven in the shape of a single spiral groove. Every small bend and tilt is part of the recording, so the stylus is not simply tracing a path—it is reading a three-dimensional pattern as the record turns.
