The Audio Video Interleave (AVI) format is known as such because of the way it encodes video and audio in blocks next to each other. To make this possible, each block is prepended with a four character identifier that identifies the codec used. An example would be DIVX for DIV3 or DIV4 for DIVX encoded video, or RAW for uuencoded (raw) video, and 0055 for MP3 (Mpeg) audio.