Even if the written instrument has been delivered, either party has the right to show any facts which prevent the writing from constituting a valid contract.1 The fact that a written contract which is not under seal has been signed and delivered, is not of itself conclusive that such contract is valid and operative for all purposes. There are various types of mistake and fraud which render a written contract invalid in spite of the outward form of signing and delivery.2 Other forms of misrepresentation and fraud, together with facts which amount to duress and undue influence,3 may render such contract voidable at law or subject to rescission in equity. Such defenses can not be interposed unless extrinsic evidence is admissible to establish the facts which amount to such defenses in spite of the fact that the contract is in writing.4 If extrinsic evidence could not be introduced, such defenses could not be interposed unless the party who was guilty of fraud, duress, and the like, had set forth such fraud, duress, and the like, upon the face of the contract itself.