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.