Another property which came from the television. The cinescopes (or CRTs, Cathode Ray-Tubes) used until the 90s did not have enough speed to write a whole picture in the screen 50 or 60 times in a second.
The solution found was to split each picture in two '''fields''' --- the first one made of all the odd lines which composed a picture, and the second one with the even lines. The human eye could not distinguish scenes shown and the materials used in the tube screens would, anyways, retain light for a little while after the light ray hit them. In such speed, the combination of the two fields would be imperceptible.
This is '''interlacing'''. Following the frequency chosen by the television engineers, 50 or 60 fields are shown per second, making for, respectively, 25 or 30 frames per second.
Later, faster cinescopes alowed the full display of pictures without the need of interlacing, and '''non-interlaced video''' allows a better quality, specially noticeable in frozen images. Sometimes, converting interlaced video to non-interlaced video or vice-versa is desireable.