twitter/bootstrap — comment on commit 102bb8e68439d28334c2c9f6e4579fdd9950c102
File:  line 0
By auzigog:

Nevermind. I see. It's more clear when you read the docs ont he page instead of the HTML itself. Since the span12 is nested withint a fixed grid already, it shows that a 12 can be within smaller spans.