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I have another question about the clause where she was the only female from the sentence Mary was working at Pete's Cafe, where she was the only female. Is the clause-which I can gather is dependent and nonessential-an adjective clause or another kind of clause? I thought it may have been an adjective clause, but wouldn't it then have to be headed by a relative pronoun? My grammar book tells me that the only relative pronouns are who, whom, whose, that, and which. Could where actually be a relative pronoun in this clause, or could the relative pronoun be implicit? I am so confused!
Comments  
It is a dependent non-defining relative clause, is where she was the only female .
When and where can introduce relative clauses after prepositional phrases (at Pete's Cafe) referring to place or time.
The preposition (at) has a modified object (Pete's is the modifier of Cafe).
The relative clause is the complement of the preposition object: Cafe.
This is how I, a self-study student, perceive things.
Is it an adverb clause? Adverb of place ? because "where she was the only female" describes the place " Pete's Cafe "as such a place where there were only men except she.I need experts' comments
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Mary was working at Pete's Cafe, where(?) (she was the only female).

at Pete's Cafe is a prep phrase in locative adjunct (adverbial) position. The relative clause is an adverbial clause and an adverb modifier.