Those who recognized the defects of the old systems, set about to correct them. In the search for a remedy, all systems in use--for whatever purpose--were carefully investigated, among others the so-called _railroad system_. For years, railroads had been keeping all correspondence relating to one subject together, attaching the letters to a strong backing sheet. The sheet, with the letters attached, was folded twice, making a package about 3½"×9" in size, which package was known as a file. These files were numbered, and kept in numerical sequence in pigeon holes, or in boxes similar to the present-day document files. When the document boxes were used, which was the case in the larger offices, the packages or files were placed on end--filed vertically. This undoubtedly was the forerunner of the present-day vertical file. The plan of keeping together all correspondence about a given subject naturally appealed to the investigator as being practicable for use in a commercial house. Not a few houses adopted the system in its entirety, which included an alphabetically indexed book in which a brief synopsis of the correspondence was written. The book was also used for a cross-index to the names of individual correspondents. It was really a system of recording the principal contents of letters, with references to the files where all of the correspondence could be found. [Illustration: Fig. 10. The Principle of Vertical Filing. _Browne-Morse Co._] While the idea of keeping all of the correspondence on one subject, or with one individual, together, was excellent, this system was not well adapted to commercial use. Writing the contents of letters in a book was entirely impractical, and too much time was required to open the files and refer to the contents. But, said someone, why not adopt the numerical idea, and substitute a numerical for the alphabetical index in the box file; instead of index sheets printed with the letters, why not have them numbered from _1_ up? Then a number could be assigned to each correspondent and all of his letters could be filed under the index sheet bearing the corresponding number. An index book could be used for the names and references to the numbers. Next came the card-index man with a proposition to use his cards for an index to the files. Not long ago the writer inspected a filing system consisting of a number of box files indexed numerically, arranged on a shelf. A card index supplied the cross-references. In this particular case the system was designed by a card-index manufacturer to create a market for card files. But the use of card indexes furnished the final solution of the filing problem. If small cards could be filed on edge, why not letters? With a larger drawer, with index guides to fit, the card index idea could be adapted to the filing of correspondence. Indeed, a flat file drawer, standing on edge, as shown in Fig. 10, illustrates the principle of vertical filing.