Principle C

Principle B is a statement in the government and binding theory of syntax that applies to referring expressions -- that is, pretty much any noun-phrase that is not an anaphor or pronoun (which are covered by principles A and B respectively).

Examples:

*She_i saw the woman_i

        S
       /  \
    NP    VP
    /    /  \
   N     V    S
 she  said  / \
           NP   VP
          /    /  \
         N    V    S
      Bill thinks / \
                NP   VP
                /    / \  
               N    V   NP
          George saluted \
                          N
                         Mary

*She_i said Bill thinks George saluted Mary_i

A referring expression must be free.

See Principle A for important context.