In writing the subject-headings difficulties will soon arise unless you follow certain general rules and are careful also to be consistent in your work. For instance, at intervals during a few months you add to the library books on horses, cows, sheep, goats, camels, and pigs; some dealing with one animal, some with two or more. If for the first one you write a subject-card with the catch-word or entry-word at the top "Domestic animals," and for the next one "Farm animals," and for the next one "Animals, domestic," you will scatter the references to domesticated animals all through your catalog, to the despair of those who would use it. You can guard against this, and easily, if your catalog is small, by looking to see what you have already written every time you write a new subject-entry-word, and by following out a previously devised plan in the making of your entries. The safest way is to get a printed list of headings and catalog rules and follow them. (See chapter on Things needed, 9.) With a printed list of subject-headings at hand it is not difficult to keep your catalog consistent and reasonable. This same list of subject-headings will serve also as a guide in the writing of the cross-reference cards for your catalog, the cards, that is, which refer the searcher from the topic "pigs," for example, to "swine," or from both to "domestic animals." Of course the subject-headings' list must be systematically used, and must be marked and annotated to fit your special needs. This work, like classifying, can best be learned by doing. There are many ways of keeping your catalog cards. The thing to use is a set of trays made for the purpose. (See Library Bureau catalog.) The cards are extremely valuable, and expense should not be spared in providing for their safe keeping and handy use. CHAPTER XXVI Preparing books for the shelves All books should be marked with the name of the library. This is cheaply done with a rubber stamp and violet or red ink pad. An embossing stamp makes a good and indelible mark. The type used should be of moderate size and open faced. A perforating stamp now on the market marks a book neatly and most permanently. Mark books freely, to assure their being recognized as the library's property wherever seen. Have some definite pages on which stamps always appear. Many use the title-page, fifty-first or one hundred and first, and the last page. This need not interfere with marking elsewhere. [Illustration: Embossing stamp.]