You can convert single-column lists into double-column lists using rectangle killing and yanking; kill the second half of the list as a rectangle and then yank it beside the first line of the list. See Two-Column, for another way to edit multi-column text.
You can also copy rectangles into and out of registers with C-x r r r and C-x r i r. See Rectangle Registers.
There are two commands you can use for making blank rectangles: M-x clear-rectangle which blanks out existing text, and C-x r o (open-rectangle) which inserts a blank rectangle. Clearing a rectangle is equivalent to deleting it and then inserting a blank rectangle of the same size.
The command M-x delete-whitespace-rectangle deletes horizontal whitespace starting from a particular column. This applies to each of the lines in the rectangle, and the column is specified by the left edge of the rectangle. The right edge of the rectangle does not make any difference to this command.
The command C-x r t (string-rectangle) replaces the contents of a region-rectangle with a string on each line. The string's width need not be the same as the width of the rectangle. If the string's width is less, the text after the rectangle shifts left; if the string is wider than the rectangle, the text after the rectangle shifts right.
The command M-x string-insert-rectangle is similar to string-rectangle, but inserts the string on each line, shifting the original text to the right.
