Television images are scanned in a sequence of horizontal lines, beginning at the upper left corner, and reaching the bottom right corner at the end of the field. Thereupon the scan is returned to the upper left corner to begin the next field. As a consequence of the line structure, all television images are sampled vertically. Within a line, the signal may remain analogue or be sampled digitally. A television line is also a measure of time, representing the interval allocated to one line. (In the U.S.: system 525/59.94/2:1, the line duration is 63.5 s). Television lines also function as a geometric measure, with resolution (both vertical and horizontal), for example, specified in terms of “lines per picture height.” Since both “black” and “white” lines of a resolution chart are counted, two television lines equal one cycle of the electrical waveform.