N.Y.C. Admin. Code § 3-507: § 3-507 Numbers in certain sections in the borough of Manhattan.

Whenever any street north of Ninth street, inclusive, in the borough of Manhattan, shall be directed to be numbered or renumbered, the president of such borough shall cause the numbers to commence at Fifth avenue, numbering east and west, beginning with number one, on the west side of Fifth avenue; number one hundred, on the west side of Sixth avenue; number two hundred on the west side of Seventh avenue, and so on, east and west of Fifth avenue, through the whole series of streets north of Ninth street, and including Ninth street. Such streets shall hereafter be called and known as East Ninth and West Ninth street, and so on, the dividing line to be Fifth avenue.