A television picture is composed of lines, of variable brightness and color. A television monitor writes horizontal lines on the screen, starting at the top left-hand corner and travelling downwards. Synchronization signals are required to tell the monitor when to start a new line (horizontal sync.), and when to flip back to the top for the start of a new field (vertical sync.). The sync. signals are in the form of electrical pulses, which are added to the video (picture) signal.