IPA: /ɡrəˈmætɪkəlnəs/
KK: /græˈmætɪkəlnəs/
The quality of following the rules of grammar in language.
The grammaticalness of her writing impressed the teacher.
Grammaticalness → It is formed from "grammatical" (from Greek "grammatikos", meaning pertaining to letters or writing) and "-ness" (a suffix meaning the state or quality of). The word "grammaticalness" refers to the quality or state of being grammatical.
Think of the quality of being related to letters or writing ('grammatical') and how '-ness' indicates a state. This helps you remember that 'grammaticalness' means the state of being grammatical.