To serve its purpose well, an index to a book must be compiled with care, the references being placed under the heading that the reader is most likely to seek. An index should be one and indivisible, and not broken up into several alphabets; thus every work, whether in one or more volumes, ought to have its complete index. The mode of arrangement calls for special attention; this may be either chronological, alphabetical or according to classes, but great confusion will be caused by uniting the three systems. The alphabetical arrangement is so simple, convenient and easily understood that it has naturally superseded the other forms, save in some exceptional cases. Much of the value of an index depends upon the mode in which it is printed, and every endeavour should be made to set it out with clearness. In old indexes the indexed word was not brought to the front, but was left in its place in the sentence, so that the alphabetical order was not made perceptible to the eye. There are few points in which the printer is more likely to go wrong than in the use of marks of repetition, and many otherwise good indexes are full of the most perplexing cases of misapplication in this respect. The oft-quoted instance, Mill on Liberty ----on the Floss actually occurred in a catalogue. But in modern times there has been a great advance in the art of indexing, especially since the foundation in 1877 in England of the Index Society; and the growth of great libraries has given a stimulus to this method of making it easy for readers and researchers to find a ready reference to the facts or discussions they require. Not only has it become almost a _sine qua non_ that any good book must have its own index, but the art of indexing has been applied to those books which are really collections of books (such as the _Encyclopaedia Britannica_), to a great newspaper like the London _Times_, and to the cataloguing of great libraries themselves. The work in these more elaborate cases has been enormously facilitated by the modern devices by means of which separate cards are used, arranged in drawers and cases, American enterprise in this direction having led the way. And the value of the work done in this respect by the Congressional Library at Washington, the British Museum and the London Library (notably by its Subject Index published in 1909) cannot well be exaggerated. (See also BIBLIOGRAPHY). There are numerous books on Indexing, but the best for any one who wants to get a general idea is H. B. Wheatley's _How to make an Index_ (1902). FOOTNOTE: [1] Another old word occasionally used in the sense of an index is "pye." Sir T. Duffus Hardy, in some observations on the derivation of the word "Pye-Book" (which most probably comes from the Latin _pica_), remarks that the earliest use he had noted of pye in this sense is dated 1547--"a Pye of all the names of such Balives as been to accompte pro anno regni regis Edwardi Sexti primo."