# File lib/prawn/document/page_geometry.rb, line 139
      def page_dimensions
        coords = SIZES[page_size] || page_size
        [0,0] + case(page_layout)
        when :portrait
          coords
        when :landscape
          coords.reverse
        else
          raise Prawn::Errors::InvalidPageLayout,
            "Layout must be either :portrait or :landscape"
        end
      end