Describes the thickness of a frame around a rectangle. Four Double values describe the Left, Top, Right, and Bottom sides of the rectangle, respectively.
Thickness values can be specified in two ways. When it is passed four Double values to the structure, they represent the Left, Top, Right and Bottom sides, respectively, of the bounding rectangle. Alternatively, it can also supply a single value, which applies a uniform value to all four sides of the bounding rectangle.