15:13 · This structure was common and widespread in ancient cultures. It has been found in prehistoric caves of Western Europe. This sign, and the following four entries, are combinations of three straight lines.
    Note that seven or eight of the letters of the Western alphabet, and one (4) or two (7 on pocket calculators) of the numerals are such three-line combinations.
    The structure is sometimes used in electrical contexts to mean light socket with switch.