RIFF files are built from

(1) RIFF Form header
'RIFF' (4 byte file size) 'xxxx' (data)
where 'xxxx' identifies the specialization (or form)
of RIFF. 'AVI ' for AVI files.
where the data is the rest of the file. The
data is comprised of chunks and lists. Chunks
and lists are defined below.

(2) A Chunk
(4 byte identifier) (4 byte chunk size) (data)
The 4 byte identifier is a sequence
of four characters such as 'JUNK' or 'idx1'

(3) A List
'LIST' (4 byte list size) (4 byte list identifier) (data)
where the 4 byte identifier is a human readable
sequence of four characters such as 'rec ' or
'movi'
where the data is comprised of LISTS or CHUNKS.

AVI is a specialization or "form" of RIFF, described below:

'RIFF' (4 byte file length) 'AVI ' // file header (a RIFF form)

'LIST' (4 byte list length) 'hdrl' // list of headers for AVI file
The 'hdrl' list contains:

'avih' (4 byte chunk size) (data) // the AVI header (a chunk)
'strl' lists of stream headers for each stream (audio, video, etc.) in the AVI file. An AVI file> can contain one video stream and zero, one, or many audio streams. For an AVI file with one video and one audio stream:
'LIST' (4 byte list length) 'strl' // video stream list (a list)
The video 'strl' list contains:

'strh' (4 byte chunk size) (data) // video stream header (a chunk)
'strf' (4 byte chunk size) (data) // video stream format (a chunk)

'LIST' (4 byte list length) 'strl' // audio stream list (a list)
The audio 'strl' list contains:

'strh' (4 byte chunk size) (data) // audio stream header (a chunk)
'strf' (4 byte chunk size) (data) // audio stream format (a chunk)

'JUNK' (4 byte chunk size) (data - usually all zeros) // an OPTIONAL junk chunk

'LIST' (4 byte list length) 'movi' // list of movie data (a list)
The 'movi' list contains the actual audio and video data.

This 'movi' list contains one or more ...
'LIST' (4 byte list length) 'rec ' // list of movie records (a list)
'##wb' (4 byte chunk size) (data) // sound data (a chunk)
'##dc' (4 byte chunk size) (data) // video data (a chunk)
'##db' (4 byte chunk size) (data) // video data (a chunk)

A 'rec ' list (a record) contains the audio and video data for a single frame.
'##wb' (4 byte chunk size) (data) // sound data (a chunk)
'##dc' (4 byte chunk size) (data) // video data (a chunk)
'##db' (4 byte chunk size) (data) // video data (a chunk)
The 'rec ' list may not be used for AVI files with only audio or only video data. I have seen video only uncompressed AVI files that did not use the 'rec ' list, only '00db' chunks. The 'rec ' list is used for AVI files with interleaved audio and video streams. The 'rec 'list may be used for AVI file with only video.

"##" in '##dc' refers to the stream number. For example, video data chunks belonging to stream 0 would use the identifier '00dc'. A chunk of video data contains a single video frame.

##dc chunk was intended to keep compressed data, whereas ##db chunk was intended to be used for uncompressed DIBs (device independent bitmap), but actually they both can contain compressed data. For example, Microsoft VidCap (video capture window class) writes MJPEG compressed data in ##db chunks, whereas Adobe Premiere writes frames compressed with the same MJPEG codec as ##dc chunks.

The ##wb chunks contain the audio data.

The audio and video chunks in an AVI file do not contain time stamps or frame counts. The data is ordered in time sequentially as it appears in the AVI file. A player application should display the video frames at the frame rate indicated in the headers. The application should play the audio at the audio sample rate indicated in the headers. Usually, the streams are all assumed to start at time zero since there are no explicit time stamps in the AVI file.

In principle, a video chunk contains a single frame of video. By design, the video chunk should be interleaved with an audio chunk containing the audio associated with that video frame. The data consists of pairs of video and audio chunks. These pairs may be encapsulated in a 'REC ' list. Not all AVI files obey this scheme. There are even AVI files with all the video followed by all of the audio; this is not the way an AVI file should be made.

The 'movi' list may be followed by:

'idx1' (4 byte chunk size) (index data) // an optional index into movie (a chunk)

The optional index contains a table of memory offsets to each chunk within the 'movi' list. The 'idx1' index supports rapid seeking to frames within the video file.

The 'avih' (AVI Header) chunk contains the following information:

The 'strh' (Stream Header) chunk contains the following information:

For video, the 'strf' (Stream Format) chunk contains the following information:

For audio, the 'strf' (Stream Format) chunk contains the following information: