It is intended to serve as a reference for accessing stories printed the previous year in the newspaper.
It was created by publisher Adolph Simon Ochs, who wanted to compete with the New York Sun by offering a series of special features.
The index, he believed, would be attractive to librarians and other researchers by making reliable source material widely available to the public.
The index was the only one published by an American newspaper through at least the 1930s, and helped make the Times known to libraries and students nationwide as a primary source for research.
In recent years, volumes of the index have been designated to cover specific months or weeks.