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What word class is 'about' in the below sentence?
She then proceeded to show me a cube about 30 cm on each side.
The dictionary says that 'about' functions as an adverb in this context. But I am well aware that the dictionary is no reliable source when it comes to this matter, and I do not trust for one moment that 'about' is an adverb in this context, seeing as its role here is completely different from the role that the vast majority of adverbs play.
Thoughts?
About modifies 30, expressing that the quantity 30 is an estimation and not exact.
Instead of about, the -ly* word approximately could be substituted and the sentence would mean the same thing. The suffix -ly is a common word ending that turns a base word into an adverb.
its role here is completely different from the role that the vast majority of adverbs play.
Adverbs modify modifiers or verbs (and tend to be a catchall category for words that don't follow the other classical parts of speech), and that's what about is doing here. 30 is an adjective.
"About" is modifying the number phrase "30 cm"
You could see this in other adverbs and similar modifying phrases: "exactly 30 cm" or "more than 30 cm".
You might insert some punctuation, a preposition or split the sentence to clarify the meaning:
a cube[, which was] about 30 cm on each side.
Adverbs are a wide class of words for words that modify. They are adjuncts that are not adjectives nor determiners nor nouns. Its therefore common to find adverbs that are not grammatically similar to other adverbs.
She then proceeded to show me a cube about 30 cm on each side.
The dictionary is right in this instance. "About" is indeed an adverb -- it's modifying the determinative "30" to form the determinative phrase "about 30".
The determinative phrase "about 30" then functions as a determiner in the noun phrase "about 30 cms on each side", which is modifying "cube".
