Interlacing of a frame is not the same as interlacing of the TV set.

	Why are the video frames interlaced? To pack 50 images (= 50 positions in time) into 25. Why not use only 25 images? To make the motions more fluid, the motion would stutter otherwise. Why not use 50 images? Because you need too much storage space or to much bandwidth, as I wrote on the Deinterlacing Homepage in "Why the heck?"
	The TV displays the odd lines first and then the even lines. This is also called "Interlacing". Why does the TV set use this interlacing? Because a) hardware technology was/is too slow to refresh 50 full sized images per second and b) originally the phosphor fade-out in video tubes was to fast. Thus a TV set displays the first image thru the odd lines and the second image thru the even lines while the the last image (the odd lines) fades out. Then it displays the odd lines again (while the even lines fade out), then the even lines and so forth. This is interlacing of the DISPLAY. This is because of the way the black TV tubes are constructed. It has to do with phosphor and with electron beams. The TV set displays its lines starting from the line at the top and ending at the line at the bottom. When it starts to display the second image it again starts at the top. At this time the first image already began to fade out (fading from top to bottom). Thus the top position of the first image is the "fade outest" when the top position of the second image is the brightest. Thus one full image as you see it on your standard TV set at any given time is a strange mix of 2 images, one fading out the other fading in. Thus the quality of the picture as you see it on your TV screen is produced by an optical illusion.

Both handicaps (little bandwidth & flickering display) go hand in hand together. They need each other to produce this optical illusion. The interlacing of the display has basically nothing to do with the interlacing of the frames of the movie. Oh yes, they are coordinated, so field 1 is displayed in the TV set's first round and field 2 in the second round, nevertheless these are 2 different phenomena of technology nowadays.

If you sent 50 noninterlaced pics/s to your TV set it would still display the odd lines after the even lines. Vice versa you could send 25 interlaced pics/s to a progressive TV set and it would display them progressively.

The confusion Interlacing vs. Interlacing is similar to Resolution vs. Resolution. There is the resolution of the frame (720x576 pixels) and the question "How many details can my TV set display?" (= "Is the electron beam too fat maybe?", "Is the sharpness adjusted correctly?", "Are noise filters set too generously?" etc.). Thus the Frame Resolution is somehow related to the Display Resolution but it's a different thing.