When it is desired to file by subjects indexed numerically, a numbered folder is headed with the name of the subject, and all correspondence pertaining to the subject is filed in that folder. An index card is then filled in with the name of the subject and the number of the folder, and filed alphabetically. Additional cross-index cards are headed with the names of correspondents, and refer to the subject and folder number. Several subject index cards are shown in Fig. 20. Subject filing can be adapted to the alphabetical index by inserting folders or guides headed with the names of subjects, as shown in Fig. 21. These folders are used for correspondence pertaining to a given subject, and cross-index cards are made for the names of individuals. =Selecting the Index.= The selection of the index for correspondence files is important, and must be governed by the nature of the business, and the correspondence to be filed. Subject indexing is the least used of any of the methods described. Railroads, and a few large corporations determine as far as possible a number of subject headings under which all correspondence shall be filed, individuals being known only in the miscellaneous correspondence; but these are exceptions, and this method of filing is not adapted to the needs of the average business concern. [Illustration: Fig. 21. Subject and Alphabetical Folder _Browne-Morse Co._] Geographical indexing is quite largely used by jobbers, and is a very practicable method for a concern whose correspondence is confined within a definite territory. It is also used to advantage in connection with other methods of indexing. For example, a concern in Chicago, with a large number of customers in Illinois, Iowa, and Wisconsin, might use a geographical index for those states, while filing all other correspondence alphabetically. This leaves alphabetical and numerical indexing from which to make a selection. A thorough investigation, which has included consultations with many leading filing experts, and examinations of the systems used by more than a hundred representative houses, from the small retail store to the largest corporations, leads to the conclusion that, except in special cases, the alphabetical index is best. It is best because simplest; and to do a thing in the most simple way is one of the cardinal principles of business system. Alphabetical indexing adapts itself to every possible requirement. Suppose, for example, that the correspondence of the Norton Machine Company is filed alphabetically, and that you want to refer to the correspondence of T. J. Watson, who is secretary of the company. A cross-index card is headed with the name of the company. This would be exactly like the cross-index shown in Fig. 16, except that the number would be omitted.