[cd34]

Virgohippy's explaination is correct, however, you can add

RewriteOptions inherit

in an .htaccess that is below a parent .htaccess and it will use the rules from both.

So, you could set up your main rules in yourdomain.com/.htaccess, and add additional rules in yourdomain.com/dir1/ with the

RewriteOptions inherit

And the files served in /dir1/ would utilize both.