In classic analog television, the television picture was displayed in two fields to reduce flicker. In the first field, all odd lines are transmitted according to the interlaced scanning method, in the second all even lines.
The successive fields with the odd and even lines together form a full picture. In the PAL television standard, the first field begins at the upper edge of the screen with half a right line and ends at the lower right edge of the screen with a whole line. The second field begins at the top of the screen with a full line and ends at the bottom of the screen with half a left line. Each field therefore has 312 and a half lines and a refresh rate of 50 Hz, which halves to 25 Hz for a full frame when there are two fields.