In the context of the keyword phrase provided, "911" functions as a proper noun. It serves as the specific name for the emergency telephone number and the associated system of emergency dispatch services in North America. The term acts as the subject of the inquiry, with "when" operating as an interrogative adverb that questions the timing or circumstances related to the noun.
A proper noun is a name used for an individual person, place, or organization, spelled with an initial capital letter. While "emergency number" is a common noun, "911" is the unique, capitalized identifier for a particular service, thus classifying it as a proper noun. In a more complete sentence implied by the keyword, such as "When should one call 911?", the term "911" would function as the direct object of the verb "call." However, as a standalone topic heading, it stands as the primary subject being modified or questioned by the subsequent term.
Understanding "911" as the core noun is critical for content development. This grammatical classification establishes that the article's central theme is the entity itself, while the adverb "when" sets the specific scope of the discussion. The article should therefore focus on defining the precise conditions, situations, and criteria that warrant interaction with the entity known as 911, directly addressing the question posed by the adverb.
