[Boost-users] MPL newbie : what is the difference between mpl::true_ and true_type

I'm a newbie for both boost and MPL. My question is regarding about the differences between mpl::true_ and true_type. While from my point of view they are both structure of constant boolean. Even in the implementation true_type is derived from mpl:true_. So why we need both data structure instead of only single one. When we should use mpl::true_ instead and true_type and vice versa Thanks