PQ codes are the first 2 of 8 sub-channel information encoded on a 'Red Book' CD. Each subcode channel is designated a letter P through W. The P channel gives simple instructions to the player about track locations. Sometimes it is used by lower cost players due to its simplicity. More sophisticated CD players will ignore the P channel and go straight to the Q channel for more detailed information. The Q channel holds all the non-audio information on a typical Audio CD. Several important player control codes like track number, track time, disc time, serial number, MCN (like a barcode), ISRC, copyright info, etc. . . CD Text info is also embedded in this channel. Channels R through W were reserved for expansion of the CD format, but have not come into use.