As the Catalogue of Printed Books in the Library of the British Museum, the first series of which was published from 1881 to 1900, was primarily arranged by author with some limited thesaural principles, and publication of the catalogue was ongoing, there was a need for a subject index, since the subject arrangement of the library was by a shelving system developed by librarian Thomas Watts, and readers were very rarely permitted access to shelves other than those in the reading room. Accordingly, librarian G[eorge] K[nottesford] Fortescue, Keeper of Printed Books, undertook the compilation of a subject index. This was first published in 1886 as A Subject Index of the Modern Works added to the Library of the British Museum in the Years 1880-1885. No attempt was ever made to compile or publish a subject index to the huge General Catalogue.
"The primary of the object of the index being to assist readers in the library of the British Museum, it was not considered necessary to reproduce such portions of the great printed catalogue (which is mainly an author catalogue) as were the nature of a class catalogue, such as the headings of Bibles and Liturgies, and for the same reason personal names were omitted as subject headings. Novels, poems, plays, and miscellaneous essays were also ignored. The index was at once found to be a great boon to readers; a second volume describing the additions of the years 1885-90 was published in 1891, and a third in 1897 for those of 1891-95. The number of entries in the third volume was about 37,760, while the total number of entries in three volumes covering the period between 1990 and 1895 was about 124,700. The three volumes were incorporated in one alphabet with the addition of the titles of books added during the years 1896-1900, aw well as the Slavonic, Hungarian, and Finnish books published between 1881 and 1900 which had not been included in the former indexes. The new edition was published as Subject Index of the Modern Works added to the British Museum in the years 1881-1900 (1902-3, 3 vols., 8vo). The edition contained about 155,00 entries in one alphabet of subjects with many sub-headings. The British Museum shelfmarks are added to the titles. . . ."(obituary of Mr. G. K. Fortescue published in The Times, October 28, 1912.
F. J. Hill, " 'Forescue': The British Museum and British Library Subject Index," British Library Journal, 1986.