They did not go to church, Maycomb’s principal recreation, but worshiped at home; Mrs. Radley seldom if ever crossed the street for a mid-morning coffee break with her neighbors, and certainly never joined a missionary circle.
I’ll just call up the principal and tell him!
If you don’t I’ll call the principal," she said.
There are no more uses of "principal" in the book.
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The principal idea is that decisions should be made by the people who are most knowledgeable about specific circumstances that impact the decision.
The singer was the principal performer; though the whole band was good.