Patent document number, i.e., the number assigned by a competent authority (typically a patent office). The number is recorded according to Recording, Use, and Display of Patent Application Data (ANSI Z39.61). The number is recorded in a YY-AANNNNN pattern where YY represents the last two digits of the calendar year, AA represents letters (when required), and NNNNN represents the variable-length numeric portion of the application number. For application numbers containing the year or a part thereof as an integral part of the number, the last two digits of the year are taken from the application number itself. For application numbers that do not contain the year, the last two digits of the year are taken from the application date. For application numbers that use the Japanese year of the emperor, the year of the emperor is converted to the Gregorian calendar. Note: An additional two positions may be added to record the complete calendar year where confusion may arise from omission of the century.