What you already know and your motivation for searching the patent literature will guide your work:
- By number. With a patent number or patent application number, finding the document should be not too hard. It should be just a matter of finding a database with the right content and matching the required format. Espacenet is a good choice. Doing a regular Google search with the title or patent number can also be successful.
- Keyword searching is appropriate for exploratory work that can be less than comprehensive or as a strategic approach to classification searching. Google Patents is a good place to do that.
- Classification searching is the formal approach to subject searching recommended by patent examiners. Espacenet - Classification Search and the U.S. Patent and Trademark Office Seven Step Strategy both offer guidance on this approach. Also, Google Patents: Advanced Search and Google Patents link to classification information.
