Sentence:  A girl, it is true, has always lived in a glass house among reproving relatives, whose word was law; she has been bred up to sacrifice her judgments and take the <WSD>key</WSD> submissively from dear papa; and it is wonderful how swiftly she can change her tune into the husband's. 
Meaning of the word marked with <WSD> tags:  In musical notation, a sign at the head of a staff indicating the musical key.