I learned that with 1080i, 480p, 480i and 720p, the number refers to the number of lines on the television screen, while the i stands for interlaced scanning, scanning every other set of lines, one set every 1/60 of a second, while the p stands for progressive scan, scanning all the lines in 1/30 of a second. I also learned that almost all current progressive scan TVs have a feature to enhance interlaced video viewing by showing all the lines in the same 1/30 of a second, then repeating the same lines a second time, so that there are no jagged edges.