+Unlike the standard region, the region-rectangle can have its corners
+extended past the end of buffer, or inside stretches of white space
+that point normally cannot enter, like the TAB.
+
+@findex rectangle-exchange-point-and-mark
+@findex exchange-point-and-mark@r{, in rectangle-mark-mode}
+@kindex C-x C-x@r{, in rectangle-mark-mode}
+When the region is in rectangle-mark-mode, @kbd{C-x C-x} runs the
+command @code{rectangle-exchange-point-and-mark}, which cycles between
+the four corners of the region-rectangle. This comes in handy if you
+want to modify the dimensions of the region-rectangle before invoking
+an operation on the marked text.
+