Constant Bitrate Encoding (CBR) is an MP3 encoding method that changes the quality level in order to ensure a constant bitrate. Whereas Variable Bitrate Encoding (VBR) encodes at the optimum quality level for each section of audio, Constant Bitrate Encoding sacrifices quality on complex sections and wastes bits on simple sections to produce a file with a consistent bitrate. Most MP3s are encoded with CBR.