A trust can override a will in certain situations. For example, if you transfer your diamond ring into your living trust, technically the trust becomes the owner of the ring, not you. So if you use your will to leave the ring to you granddaughter, but you name your niece to get it through your trust, your trust would override your will because your trust, not you, owned the ring when you died. There are some exceptions to this general rule. See a lawyer if you have a specific question.