 One field contains all odd-numbered lines in the image; the other contains all even-numbered lines.A Phase Alternating Line (PAL)-based television set display, for example, scans 50 "fields" every second (25 odd and 25 even).The two sets of 25 fields work together to create a full "frame" every 1/25 of a second (or 25 frames per second), but with interlacing create a new half frame every 1/50 of a second (or 50 fields per second).