After September 11, 2001, clothing with "NYPD" and "FDNY" became popular sellers. Both sets of initials were trademarked only vary recently.
Why do the agencies have these initials? Why not NYPD and NYFD, or FDNY and PDNY? Good question. Certainly, it's easier this way to tell the initials apart.
Actually, the Fire Department had historically used NYFD. Trademark records indicate that FDNY has been used since 1898 (when the NYC boroughs joined as a city). The unofficial website below claims an 1870 date for "FDNY."
The Metropolitan Fire Department continued to be the object of politics. On April 5th, the "Tweed Charter" was passed in Albany. The Tweed Charter abolished all state control over New York City. Among other provisions, it wiped out the Metropolitan Fire District, established a new Board of Fire Commissioners and the Metropolitan Fire Department became the "Fire Department of the City of New York". This act, indirectly, was responsible for the "F.D.N.Y." logo on apparatus instead of "N.Y.F.D.". Most departments place the city or town initials prior to "F.D." but as a direct carry over of the provision to create the "Fire Department of the City of New York", "F.D." was placed before "N.Y." on the apparatus, a tradition which lasts today.
On May 21st, the new Board of Fire Commissioners ordered the removal of "MFD" from all apparatus to be replaced by "F.D.N.Y."
In another part of the room was a unique design in gas-work, representing the varieties of apparatus employed by the Department, with the letters "N. Y. F. D." above it, also in gas-work.
The writer of the postal states that the touring car he saw bore a Fire Department sign, presumably "F. D. N. Y.," on his front and rear plates.
Description of Mark The mark consists of the initials "NYPD" and the years "1845-1995" superimposed on the initials. Above the initials is a drawing of an eight-pointed police badge as used by the New York City Police Department in 1845. Below the initials is a drawing of a current police badge as used by the New York City Police Department. The numeral "3100" is arbitrary and not intended to identify any individual police officer. All of these elements are contained in a stylized police officer's badge on which is perched an eagle with outstretched wings. The lining and stippling shown in the drawing are features of the mark and are not intended to indicate color.