Rules of grammar fall under the second meaning. In other words, scholars who study the language look at the patterns that generally prevail in the language and describe those patterns.
However, since " correct grammar" is often linked to one's success in modern life, many people see grammar rules in the first sense of the word; that is, as prescriptive rather than descriptive.
The English language is a living and growing thing. Like all living things, it changes. As the language changes, some of the descriptive rules change with it. We can see how the descriptive and prescriptive rules come about by looking at the way dictionaries are put together.
For example, in the 1969 edition of the AHD, ninety-five percent of the Usage Panel considered the following structure incorrect: " Nobody thinks the criticism applies to their own work." They considered this incorrect because the pronouns " nobody" and " their" both refer to the same groups of people. However, the first pronoun is singular, whereas the second is plural. Language does change. In the 1996 edition of the dictionary, sixty-four percent of the panel found the following sentence acceptable in informal speech: " No one is willing to work for wages anymore, are they?" In that sentence " no one" is singular, while " they" is plural.
(See more about this at pronoun agreement.)
Also see stupid rules