What is the contractual relationship between a surety and the trustee?

Two old English cases outline these principles. Lord Westbury discusses the contractual relationship in Blest v. Brown (1862), 4 De G.F. & J. 367; 45 E.R. 1225, at 1229. He says: "It must always be recollected in what manner a surety is bound. You bind him to the letter of his engagements. Beyond the proper interpretation of that engagement you have no hold upon him. He receives no benefit and no consideration. He is bound therefore merely according to the proper meaning and effect of, the written engagement that he has entered into. If that written engagement is altered in a single line, no matter whether it be altered for his benefit, no matter whether the alteration be innocently made, he has a right to say: 'The contract is no longer that for which I engaged to be surety, you have put an end to the contract that I guaranteed, and my obligation therefore is at an end'."