The horizontal sync signal is a short pulse at the beginning of each video line which keeps the horizontal scanning of the monitor exactly in step with the transmission of each new line. The vertical sync signal is a pulse transmitted at the beginning of each field and frame. Its purpose is to maintain the monitor in field-by-field synchronization with transmission of the next frame pulse. Sync signals reside in a part of the video signal in which no visual picture information is transmitted. During that particular part of the transmission (the blanking retrace period or vertical interval), the electronic beam is blanked and retraces back to the other side of the screen to start a new line or a new frame. Both horizontal and vertical sync are needed to have a fully stable picture.