close as the bark to the tree

As connected as is possible. Often used to describe a particularly intimate relationship or friendship. Jenna and Elise are as close as the bark to the tree—I rarely see one without the other! I thought we were as close as the bark to the tree, so I was completely blindsided when David broke up with me. Yeah, Emma and I are friends, but I wouldn't say we're close as the bark to the tree. I'm closer friends with Sophie, actually.