All grammar syntax rules (afaik) pertain to words in the same sentence. For example, a complete sentence must have a subject and a verb. But there must be rules for structures larger than the sentence. For example, take the following sentence:
We may put this in another way.
The reader knows that the sentence refers to something written previously. But would the writer be breaking a grammatical rule if he started an essay with this sentence? He would have to be breaking a rule of some sort -- but if this rule is not grammatical, of what sort is it?