Since the right edge of the screen has now been reached, there are no more pixels off in that direction. It is implicit that the next bytes read from memory will be for the leftmost pixel in the second raster-line down from the top. If this is a cathode-ray tube type of screen, we have a little timing problem here in that the electron beam is currently at the right edge of the screen and now it’s being asked to draw a pixel at the left edge. It has to move back. This takes a little while—not long, but much longer than the interval of time between drawing two pixels that are cheek-by-jowl. This pause is called the horizontal retrace interval . Another one will occur at the end of every other line until the rastering has proceeded to the last pixel at the bottom right-hand corner of the screen and completed a single grave-rubbing. But then it’s time to begin the process all over again, and so the electron beam (if there is one) has to jump diagonally all the way up to the upper left-hand pixel. This also takes a little while and is called the vertical retrace interval .