All two-channel or stereo recordings have a left, L, and right, R, audio channel. These channels do not contain the same audio information. Most studio recordings are really multi-channel mono recordings, in which the producer 'places' the audio signal from each channel somewhere between hard left and hard right using the pan control on the mixing console. Each instrument or voice is recorded very close to the source, eliminating any room effects. The relative content, left vs. right, is a production decision, so listening with the left earphone in your left ear and the right in your right reproduces the environment the producer was trying to create. In real stereophonic recordings (stereo = solid in Greek), at least two microphones, usually oriented towards the left and right sides of the hall, are used to record a musical performance from the listener's perspective. The audio information that is recorded includes the musical source material, as well as any room reflections and any other out of phase sounds. When this is played back in the proper orientation - left channel to the listener's left and right to the right - the performance can be reproduced with many, if not most, of the directional cues intact. Playing with the channels reversed can ruin the acoustic image.

they can be used in both ears, but they fit more comfortably when you put the 'L' in your left ear and the 'R' in your right ear.

they have left and right symbols as the music you are listning to is masterd (ie mixed by the sound producer for optimum sound from left and right thats why. try listening to a song with left in right ear and right in left ear i bet you hear a distinct difference. hope this helps x