Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 435)

Movie Playback
1.
Switch to playback.
Press the < > button.
2.
Select a movie.
Turn the < > dial to select a movie to play.
In single-image display, the < > icon displayed in the upper
left of the screen indicates a movie.
In index display, perforations at the left edge of a thumbnail indicate a
movie. Movies cannot be played back from index display, so press
<
> to switch to single-image display.
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