When choosing a class number, it should represent (as nearly as possible) the primary subject(s) of the material being cataloged, as modified by a topical subheading when appropriate, or, in some cases, by the form or type of material being cataloged. Use the most specific number available.

After assigning the MeSH terms to a work:

1. Search the primary subject(s) in the Index of the NLM Classification.
2. Verify numbers selected from the Index in the main schedules because there may be an appropriate specific class number that is not listed in the index to the classification.
3. Read all annotations in the schedule (Note: Some general annotations appear at the beginning of the schedule).
4. Consider factors that may influence the classification (e.g., form, age group).

Use a broader number if:

• No specific number is available.
• The work deals with several subjects that, taken together, constitute a major part of a larger subject.

If more than one primary subject is considered, choose the class number using the following guidelines:

• Follow the general principles as covered in this training (e.g., LCC vs. NLM Classification).
• Check for special instructions in the schedules.
• If no subject is dominant, class under the first one mentioned in the work being cataloged.