Angelina Jolie has praised Billy Bob Thornton's ''big, beautiful heart''.

The 'Salt' actress - who raises six children with fiance Brad Pitt - also praised her second husband's sense of humour when penning the foreword to his new autobiography 'The Billy Bob Tapes: A Cave Full of Ghosts'.

She wrote: ''He has an unmatchable wit and can make you laugh until your face hurts and he has a big beautiful heart.''

Angelina - who split from Billy Bob in March 2002 after almost two years of marriage - also described the actor as a ''mad mathematician'' and admitted one of things she loves most about him is his eccentricity.

She added: ''To be with him is like being with a mad mathematician. He is constantly counting and repeating.

''Some people walk through life able to quiet the voices in their heads. He can't. And I, and everyone else who knows him well, we love him for it. I know one thing: The world would certainly be a hell of a lot more dull if that man weren't in it.''

Billy Bob recently admitted he and Angelina - who has also previously been married to Jonny Lee Miller - split because he felt he wasn't ''good enough'' for her.

He said: ''We had a great marriage and I chickened out because I didn't feel good enough. That's all that happened. It was no big deal, we never hated each other.''