When dealing with interlaced video, you are viewing a number of fields per second. Progressive video is comprised of frames per second, which contain every odd and even scanline of video. Interlaced video displays all the odd scanlines first, followed by all the even scanlines second. These collections of odd and even scanlines are called fields. Think of a full picture frame, scanlines 1, 3. 5, 7, 9 etc. would be displayed in the first field and scanlines 2, 4, 6, 8, 10 etc. would be displayed in the second field. PAL video runs at 50Hz (50 fields per second) and NTSC runs at 59.94Hz (59.94 fields per second).