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In CVS, branches and tags are first class entities – CVS contains several commands for manipulating these. Subversion does not. Instead, Subversion's support for cheap file/directory copies within the repository is used to support the notions of tagging and branching.
A number of conventions have developed to support this. These conventions are not enforced by any of the base Subversion tools, but they can be enforced by pre-commit hooks in the repository (if it is felt that these decisions need to be programmatically enforced).
trunk/, tags/, and branches/
The most pervasive convention