How is the width and height of x,y,w,h=boundingRect(c) determined?

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I have looked everywhere online, and everywhere I look I see that boundingRect outputs the x,y positions of the top left corner of the contour and the width (w), and height (h) are determined from that point. I want to know what determines what the width and height are? How does boundingRect() work? If that information is online somewhere, I have not been able to find it using google.