Every frame of non-progressive scan video is composed of two interleaved fields. The result is either 50 fields per second for PAL or roughly 60 fields per second for NTSC. Interleaving means that all of the even lines are in one field, and all of the odd lines are in another field. The terms "even" and "odd" field are only useful for illustration purposes. These terms, however, are not accurate when discussing field order--is the first line 0 (even) or 1 (odd)? Rather, it is best to specify "upper" or "lower" field first when specifying field order. DV is lower-field first.