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   The third normal form involves each field of the file. This rule
   describes, in other words, the way fields are to be distributed among the
   files.

   A file is in third normal form if the value of each of its fields depend,
   in a non-transitive way, from the value of each of its keys.

   A field A has a transitive dependence on a key C if there
   exists a field B in such a way that B depends from C and A
   depends from B.

   Consider the following example:

   Let's suppose that the file Art contains the fields ArtCode, ArtDisc and
   ArtCat. These fields respectively describe the code, the discount and the
   category of the article. The primary key is made up of the value of the
   ArtCode field and the file is in second normal form.

   The discount rate of the article depends, however, on the category of the
   article since we assume that all the articles of a given category are sold
   with the same discount rate.

   This means that the value of the field ArtDisc depends on the value of
   ArtCat, which depends from ArtCod. In fact, the article code uniquely
   identifies the category (different articles with the same code do not
   exist), while the article's category uniquely identifies the discount
   (different articles of the same category do not have different discounts).
   Therefore, a transitive dependence exists.

     ArtCode ---> CatCode ---> Discount

   In order to avoid this transitive dependence, a second file must be
   defined, in which the "middle" field (CatCode) becomes the primary key,
   and move all fields that depend from CatCode to the new file.