Thank you!  I just Googled the variable you pointed me to 
(split-width-threshold), and instantly turned up the Emacs 23.3 page at 
http://www.gnu.org/software/emacs/elisp/html_node/Choosing-Window.html, 
which describes the function display-buffer. display-buffer is described 
as "the basic facility that chooses a window to display a buffer in."  
This sounds like exactly the function I was looking for.  The 
documentation shown when I run M-x describe-function display-buffer in 
21.3 doesn't contain any mention of a variable named 
split-width-threshold, which is pretty strong evidence that that feature 
wasn't added until some later version.

    Mark

On 8/23/2011 7:34 AM, Bernardo wrote:
>
>> After I type "o" it used to look like:
>>
>>     +-----------------------------------------+
>>     |  Dired buffer                           |
>>     |  ...>file #1                           |
>>     |  ... file #2                            |
>>     |  ... file #3                            |
>>     |  ... file #4                            |
>>     |                                         |
>>     +-----------------------------------------+
>>     |  This window now displays the contents  |
>>     |  of file #1.                            |
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     +-----------------------------------------+
>>
>> BUT NOW it looks like:
>>
>>     +--------------------+--------------------+
>>     |  Dired buffer      |  This window now   |
>>     |  ...>file #1      |  displays the      |
>>     |  ... file #2       |  contents of       |
>>     |  ... file #3       |  file #1.          |
>>     |  ... file #4          |                    |
>>     |                    |                    |
>>     +--------------------+--------------------+
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     |                                         |
>>     +-----------------------------------------+
>>
>
> setting the value of split-width-threshold variable to a larger value 
> might help;
> maybe something like this will make a difference:
> (setq split-width-threshold 300)
>