each frame (in an interlaced system "i") has 2 fields the first field or "upper" writes odd lines: 1, 3, 5 etc the second field or "lower" writes all the even ones. the linesin your sample pic show the movement between field 1 and 2 as interpreted on a progressive system. there was agood argument for fields as it gave better "movement resolution"- 50 captures of movement per second instead of a more stuttery 25 (in pal)
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