Definition:Rule of Formation

Definition
Let $\mathcal L$ be a formal language whose alphabet is $\mathcal A$.

The rules of formation of $\mathcal L$ are the rules which define how to construct words in $\mathcal L$ which are well-formed.

That is, the rules of formation tell you how to build strings consisting of symbols from the alphabet $\mathcal A$ which are part of the formal language $\mathcal L$.

The rules of formation of a formal language together constitute its formal grammar.

There are no strict guidelines on how a rule of formation should look like, since they are employed to produce such strict guidelines.

Thus, these rules of formation are often phrased in natural language, but their exact form is to some extent arbitrary.

Also see

 * BNF Specification of Propositional Calculus, in which one can see rules of formation employed.
 * Bottom-Up Specification of Propositional Calculus, a different approach to rules of formation.
 * Formal Grammar