How do you control whether modules are vertically or horizontally arranged?

The optional style="" attribute is available for the module and modules types of <jdoc:include /> statements. The attribute value refers to the "chrome" style used to wrap the output generated by a module. If no style is provided, a value of "none" is used by default. The standard styles which are declared in /templates/system/html/modules.php include:

table
horz
xhtml
rounded
outline

Template designers may add additional chrome names by adding a custom version of this file to /user-template/html/modules.php.

For more information on module positions in templates, see jdoc statements.

How do you control whether modules are vertically or horizontally arranged?

From Joomla! Documentation