Television lines (TVL) is a specification of an analog camera's or monitors's horizontal resolution power. It is alternatively known as Lines of Horizontal Resolution (LoHR) or lines of resolution. The TVL is one of the most important resolution measures in a video system. The TVL can be measured with the standard EIA-1956 resolution chart.
TVL is defined as the maximum number of alternating light and dark vertical lines that can be resolved per picture height. A resolution of 400 TVL means that 200 distinct dark vertical lines and 200 distinct white vertical lines can be counted over a horizontal span equal to the height of the picture. For example, on 4 by 3 inches (10.2 cm × 7.6 cm) monitor with 400 TVL, 200 vertical dark lines can be counted over 3 inches (7.6 cm) width on monitor (Note that the 3 inches (7.6 cm) of monitor height is used rather than the 4 inches (10 cm) of whole monitor width).
TVL is an inherent quality of a camera or monitor and should not be confused with the horizontal scanning lines of broadcast television systems, which e.g. for a PAL system are 625 lines, and for the NTSC system 525 lines.