On a CRT, the electron beam moves from top to bottom in a smooth continuous motion, then jerks quickly back to the top, about 60 times per second. The beam moves left to right in a smooth continuous motion, then jerks quickly back to the left, about 15,750 times/second. The jerk back to the left will occur if the start of a sync pulse is received roughly 60-70 microseconds after the previous horizontal jerk. The jerk back to the top will occur if a sequence of long sync pulses are detected 16-17 milliseconds after the previous vertical jerk. The electronics driving the CRT don't care about concepts like even and odd frames. They simply performs the vertical and horizontal motions, independently, as requested by the sync pulses. When a horizontal sync pulse is received, the vertical position of the beam will depend upon the amount of time since the last vertical sync pulse.