 The lines are numbered in the order in which they are scanned (so it is incorrect to talk of the 'odd numbered lines' and the 'even numbered lines' when referring to interlaced video - but see PsF Line Numbers).In 525/60 systems, by convention, the first field in the frame is considered the even field.In 625/50 systems, by convention, the first field in the frame is considered the odd field.