720x480 is the digital format that works for either 4:3 or 16:9 images. Analog NTSC SD standard is 640x480. DVD's are downsized for analog SD TV's. 720x480 is the digital standard, but 640x480 is the old analog standard. There is also 704x480, which was used on some early DVD's, but I think it is defunct now. I think there were other formats, but 720x480 (or 720x576 for PAL/SECAM) has won out over just about everything else. I have been doing this long enough to remember when 720x480 didn't exist.