'Kate Middleton is not of royal descent' ?

If you or I were able to trace our families back 500 - 600 years, we could surely find a few members of royalty. And we would find that we are related to practically everyone else in the world. But in the real world, we are not royal, and we are not related. . Kate is from a normal middle-cl family, that is why she is called a commoner. She really is a "normal" girl with no royal relatives.