+@findex rectangle-mark-mode
+ The command @kbd{C-x @key{SPC}} (@code{rectangle-mark-mode}) toggles
+whether the region-rectangle or the standard region is highlighted
+(first activating the region if necessary). When this mode is enabled,
+commands that resize the region (@kbd{C-f}, @kbd{C-n} etc.)@: do
+so in a rectangular fashion, and killing and yanking operate on the
+rectangle. @xref{Killing}. The mode persists only as long as the
+region is active.
+
+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.
+