Changing street numbers, room numbers, or postoffice box numbers, is another common method of keying. This can be operated successfully by notifying the local postal authorities that all mail is to be delivered at one number, regardless of the address. An adaptation of the number key, which can be used to advantage for a small number of publications, is a combination of numbers representing publications and dates. First, the publications are numbered, these numbers being used as the first part of the address. To the publication number is added the number of the month or week of publication. To illustrate: Suppose that the Technical World is No. 5 on the list and the ad is run in October. The key number would be 510--5 standing for the publication and 10 for the month. All replies with that address would be credited to the Technical World for October. In using this system it is necessary to use some figure--usually 0--for a repeater to avoid confusion; No. 112 might mean publication No. 1, month No. 12, or publication No. 11, month No. 2. By adding a naught after the publication No., confusion is avoided--1102 would mean No. 11, 2nd month, 10012 would indicate No. 10, 12th month. The system can be further varied by using N., S., E., and W., or by substituting Ave. for St. or _vice versâ_. When readers are requested to ask for catalogs or other printed matter, the numbers by which these are known to the reader can be used for the key--as bulletin _A_, bulletin _B_, etc. For either direct inquiries or orders one of the most popular and satisfactory keying systems is to use a coupon, indicating the publication and month in the coupon--_T W 11_ printed in small type in the coupon would mean Technical World for November. [Illustration: Fig. 16. Reverse of Checking Card Used for a Record of Sales] A coupon to be filled in offers the reader a convenient means of answering the ad, and this has a tendency to induce a larger number of replies. Few people will fail to use the coupon or the address given. A certain very large advertiser, who uses the coupon, states that less than one per cent of his replies are without means of identification; either the coupon is used or the name of the magazine is mentioned. To make their checking systems as nearly perfect as possible, some advertisers write to all readers who fail to use the key, requesting them to state where the ad was seen. Some enclose a postal for the reply, others enclose a printed slip, on which the magazines used are listed, to be checked and returned.