650 - Topical Subject Heading

The 650 tag is where you will find subject access points (from various thesauri as identified in the 2nd indicator) that are topics. This can include:

• A general subject term -- Chemistry
• The name of an event -- Anglo-Egyptian War, 1882
• Names or terms applied to individual objects or classes of objects -- JavaScript (Computer program language)
• Systematic names of chemical compounds -- Dihydrogen monoxide
• Systematic names of families, genera, species in botany and zoology -- Sepiolidae  
• A geographic name included in a phrase subject heading -- Iran in the Qur'an

The Library of Congress maintains an Alphabetical List of Ambiguous Headings which tells a cataloger what tag a particular kind of topic belongs in. For example, Celestial bodies (like Halley's comet) belong in an X51 tag because they are considered geographic headings rather than topical ones.

The subject heading will be found in subfield a. There are several additional subfields where you will find information that further specifies the topic:

• Subfield v contains form subdivisions (Statistics, Maps, etc.) -- identifies the kind of information the item contains
  
  
• Subfield x contains general subdivisions -- this is a more specific aspect of the topic in subfield a
  ( ǂa Automobiles ǂx Motors ǂx Cylinders ǂx Fluid dynamics, or ǂa Zombies ǂx Religious aspects ǂx Buddhism)
  
  
• Subfield y contains chronological subdivisions -- what year or time period the work is covering (1971-1980, or 20th century, or Edo period, 1600-1868)
  
  
• Subfield z contains geographic subdivisions -- where the thing in subfield a is located (ǂa Libraries ǂz New Hampshire)

If the work as a whole is about a place (like New Hampshire) rather than being about a specific topic (like libraries) in that place, it should have a 651 tag to represent that geographic topic. Next week I will talk about that tag.