Pictures are made of 405 lines ( BBCI and ITA ) . It means the electron beam scans across screen in 405 lines from top to bottom. It does this 25 times every second. It scans odd lines first, e.g. 1, 3, 5, 7, 9, 11 and so on up to 20  1/2 times. It then switchhes back to the top of the screen and covers the left out lines, i.e. 2, 4, 6, 8, 10 etc. It thus forms two interlacing frames of 20  1/2 lines each, making up the full number of 405. BBC 2 uses 625 lines, and the extra lines improve the picture quality making it smoother.