Answer:
- Grammar.
Step-by-step explanation:
Grammar is demonstrated as the entire system or methodology of a specific language and its rules as well as principles that define how the syntax and morphology together contribute to the formation/construction of a language.
As per the question, grammatical rules proposes that while beginning a sentence a dependent clause or introductory phrase it requires a comma that 'sets off the stage' for principal or independent clause by separating the subordinate clause(through a break provided by the comma) and make the central meaning of the sentence more clear and evident. Thus, this is the rule of 'grammar'.