Why does span6 move out if he or span3 put a border (that is, between them)?

Question

css

There is a bootstrap grid, in the span9 diva there are two span3 and span6 divs. Why does span6 move out if he or span3 put a border (that is, between them)?

Answer 1

The first answer in this service, I can not confirm, but when you put a border of 1px for example, this 1 pixel is made not inside, but outside the div, thereby violating the width. Because of this, everything moves out.
Just inside for example span6 create a div and already assign a border to it. I hope answered.