US Patent Classification Manual
classes arranged by art unit

When searching patents sometimes, it may be useful to search by class and subclass in
addition to searching based on keywords.  Of course, to search by classification you
need to know where the invention you are searching for is classified.  For older patents,
when searching U.S. patents, you may want to refer to the US Classification Manual.  
Each class and subclass has a definition (although some times the definitions are a bit
obtuse), which can be found by clicking on the class or subclass in the online version of
the US Classification Manual. Finding a class in the classification is somewhat
cumbersome, because sometimes it may be difficult to figure out which class a particular
subject may be found if there is no class with that title.  However, that is what the
classification index is for.  The classification index lists topics in alphabetical order
according to subject. So you may want to consult the classification index before
consulting the Classification Manual.  The US now uses the International Patent
Classification system, and so you would need to consult the corresponding manual, also.
The International Patent classification is also useful when searching PCTs and European

After determining a class and subclass where you believe you invention is located, you
may want to ask the Examiner that examines that subclass for searching advice.

The classification system is hierarchical.  Under each major subclass one can generally
find indented subclasses, which are subcategories of the major subclass.  In the US
classification system, when two subclasses are at the same hierarchical level, the one
that is higher up in the hierarchy is more general than the one lower than in the
hierarchy.  The classes and subclasses generally classify devices according to their
structural features and classify methods according to their steps.  In general the
classifications are not based on the usage.  So, two devices that have nearly the same
structure should classified the same subclass even if the two devices are used for totally
different purposes.  More information about the classification system may be found on
the website of the US Patent and Trademark Office.
Some links that are related to searching
that you may want to consult are: