Is it valid in subversion to branch a subfolder?

I have a subversion project that have a repository and trunk, tags, branches top-level items. For this project, trunk contains a folders for a big parts like 'common', 'libs', 'core', 'help', 'install', 'tests' etc. The question: is it valid in subversion to branch trunk/common or do i need to always branch entire trunk?

I have checked - all seems work fine while branching trunk/common, but TortoiseSVN开发者_StackOverflow中文版 don't show such branches in revision graph :(. This leads me to the assumption that it is not correct to branch something over than trunk.

You can branch every directory in SVN but it's a best practice to branch from trunk. The result in tortoise is cause TortoiseSVN assumes branches are made from trunk folder instead of subfolders. The questions is why do you try that an what advantages do you expect from "branching " a sub-folder instead of trunk?