When shooting a movie, a director creates a storyboard. A comic book artist must do the same thing for drawing a comic book. Thumbnails are small blocks that represent the pages in a comic book. There is no finished work, but sketches. When following the script, the artist needs to be aware how many words are in each frame. The publisher will not be shy about putting the words on top of the drawing, or yelling at the artist. The thumbnails allow the artist to pay close attention drawing varied frames. Don't worry about it looking pretty, but the layout is the biggest concern.
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