The index may be numerical, alphabetical, or geographical. When the numerical system is used, a card index furnishes the cross-reference. The card contains the name and the number of the folder, with a brief history of the customer's dealings with the house. FILING HELPS =Guiding.= The importance of guiding should not be overlooked, for guides are the great essential of a successful filing system. They must be simple and correct. They must be inserted at sufficient intervals to guide eye and hand instantly to the desired folder. A vertical file drawer holds about 5000 letters, and, assuming that the average is 10 letters to a folder, this would mean 500 folders. A general rule is to use 50 guides to a drawer, or one guide for every ten folders. The estimate of the number of guides required should be liberal. If there are 500 regular correspondents, it is better to equip the files with a 60 subdivision alphabetical index than to confine it to a 50 subdivision index. Guides must stand hard usage and remain legible; therefore the best quality should be used. All manufacturers of filing equipment now supply guides of heavy pressboard with metal tips. The metal tip guide is entirely rigid and supports the papers or folders in the file, keeping them rigid and in alignment. An important feature is that the index heading (name, letter, or number) can be changed at any time by merely inserting a new slip in the metal tip. Metal tip guides cost more than the old-style manila guide stock, but in the end are more economical. The money paid for a filing outfit should be regarded as a permanent investment; if the expense is to be cut down, let it be in the purchase of supplies used for transfer, rather than in the regular files which are used every day. =Transferring.= Correspondence which is out of date should be removed from the current file and filed in transfer files or boxes, indexed as in the regular files. Transfers should not be made too frequently. It is often advisable to use cabinets large enough to hold the correspondence for two years, one part being used for current correspondence, the other for correspondence one year back. At the end of the year, all of the correspondence is removed from the older file, which becomes the current file during the succeeding year. [Illustration: Fig. 27. Sorting Tray] =Sorting.= The work of the file clerk is facilitated and greater accuracy insured by the use of a sorting tray, as shown in Fig. 27. This consists of a wooden tray, equipped with alphabetical or numerical guides according to the system used. For the alphabetical system a set of _A_ to _Z_ guides--one for each letter of the alphabet--is used. Before attempting to file, the clerk sorts the day's correspondence in this tray. All correspondence belonging in one division of the alphabet is thus brought together, and can be quickly filed. SELECTING FILING EQUIPMENT