- Set mark at current cursor position:

  Ctrl-Space

- Define opposite corner of rectangle:

  Move cursor to position

- Fill current rectangle with blanks:

  Ctrl-X,R,O  (open rectangle)

- Erase current rectangle:

  Ctrl-X,R,K  (kill rectangle)

- Copy current rectangle into internal buffer:

  Ctrl-X,R,R  (register rectangle)

- Paste (yank back) rectangle from internal buffer:

  Ctrl-X,R,Y  (yank rectangle)