An NTSC video image consists of 525 horizontal lines of information. The electron gun scans top to bottom, left to right, odd numbered lines first, then the even numbered lines. Each full scan of even numbered lines, or odd numbered lines constitutes a "field". Each field scan takes 1/60th of a second, therefore a whole frame is scanned each 1/30th of a second. (literally 29.97 frames per second)

Film is generally shot and projected at 24 frames per second (fps), so when film frames are converted to NTSC video, the rate must be modified to play at 29.97 fps. During the telecine process, twelve (12) fields are added to each 24 frames of film (12 fields = 6 frames) so the same images that made up 24 frames of film then comprise 30 frames of video.Video plays at a speed of 29.97 fps so the film actually runs at 23.976 fps when transferred to video.

The Avid Film Composer assumes a 2-3 pull down. That means that the first frame of film is represented by 2 fields of video; the second frame of film is represented by 3 fields of video (1.5 frames); the third frame of film is again represented by two ields and the fourth frame of film is represented by 3 fields, and so on. In the end, what was running at 23.976 fps is running at 29.97 fps.

The first frame of video contains two fields of the 1st (A) frame of film.

The second frame of video contains two fields of the 2nd (B) frame of film.

The third frame of video contains one field of the 2nd (B) and 3rd © frames of film.

The fourth frame of video contains one field of the 3rd © and 4th (D) frames of film.

The fifth frame of video contains two fields of the 4th (D) frame of film.

The graphics above shows how four frames of film become five frames of video; repeat that process six times and 24 frames of film become 30 frames of video. (technically, 23.976 frames of film become 29.97 frames of video, but it is easier to speak in whole numbers)

The Avid digitizes (records) and plays the film at 24 fps, in a Film Project, so the video has to be stripped of the fields that were added in the tape transfer process. Systems that digitize the 29.97 frames of video produce film Cut Lists by a process called matchback where the timecode (from an EDL) is used to locate the nearest real film frame for the negative cutter. Matchback is only accurate + or - one frame. One can choose to work at 30fps on an Avid and matchback for a negative cut, or work at 24 fps and produce a frame accurate negative Cut List. There is a process by which one can import an EDL from a 24 fps film project into a 30 fps project and redigitize the picture at a higher resolution (film projects only capture in single field resolutions).

It is commonly referred to as 3-2 pulldown; while modern telecine machines can go either way, the norm is 2-3. Therefore, AA BB BC CD DD. If the telecine is set for 3-2, you'll get BB BC CD DD AA, which would require you to change the default pullin before digitizing the clips, because the clips head frames would be "B" rather than "A".