Definition:Backus-Naur Form/Rules of Formation

 Let  be a metasymbol of an object language.

Let,  , ...,   be letters of Backus-Naur form.

That is,, ...,   may be either metasymbols of, or words in, the object language.

The rules of formation of Backus-Naur form specify two kinds of well-formed formulae:



This means that  on the LHS may be replaced by the specified sequence of $n$ words on the RHS.



This means that  on the LHS may be replaced by one of the $n$ words on the RHS.