Classification searching, which is based on the pre-digital era filing system of the US Patent and Trademark Office, has a bit of a learning curve.  But once you master it, you may find that it is an efficient and thorough way to navigate the patent databases.

Plan your search!

1. What are the "key words" in your search?  Which terms get to the heart of the invention/technology?
2. Think of at least three synonyms.
3. Think of alternate spellings.
4. Could your technology have applications in different fields?  What terminology is used in those other fields?

If your patent number search is not yielding any results, run through this mental checklist:

• Am I looking for the right type of document?
• Patent application numbers tend to include a year (2007123456), whereas patents are just accession numbers (12345667).
• Am I looking in the correct database?
• If a citation does not have the country prefix, don't assume it's a US patent.  Check one of the databases that has international coverage.
• Am I entering the number using the correct format?
• Some patent databases require you to "pad," or apphend, extra zeros to a patent number to make it a specified number of characters.  For example, US patent 123456 might need to be entered as 0123456 to make seven digits.
• Is my citation correct?
• Hey, it happens.  If you think you have a bad cite, is there any other info you can go on?  See the sections on retrieving patents by name or institution.