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Dewey Decimal Classification System

In 1876 the American librarian Melvil Dewey published the first edition of the Dewey Decimal Classification system (DDC). It has been periodically updated and is still the main form of classification for library items. The 22nd edition is currently in use.

000    General knowledge
100    Philosophy, paranormal phenomena, psychology
200    Religion
300    Social sciences
400    Language
500    Natural sciences and mathematics
600    Technology (Applied science)
700    The arts. Fine arts and decorative arts
800    Literature and rhetoric
900    Geography and history 

Each main class has ten divisions, for example:

300 Social sciences
310 Statistics 
320 Political science
330 Economcs
340 Law

Each division in turn breaks down into ten sections, for instance:

340 Law
341 International law
342 Constitutional and administrative law
343 Law in military affairs, defence, public property, finance, taxation, commerce and industry 

The first three numbers may be followed by a full stop, and then by other numbers describing the subject of the book more specifically:
341.2422  International Law. European Union