Broadcast television systems followed ITU standards from 1936 with the A standard of 405 lines up to the N standard in 1951 with 625 lines.
A 1936 405 N 1951 625 M L K' K J I H G F E D C B On an international conference in Stockholm in 1961, the International Telecommunication Union designated standards for broadcast television systems. Each standard is designated a letter (A-M); in combination with a color system (NTSC, PAL, SECAM), this completely specifies all of the monaural analog television systems in the world (for example, PAL-B, NTSC-M, etc.). The following table gives the principal characteristics of each standard. World analog television systems 
