It is said by Lord Coke 6 that a deed must be written on paper or parchment, but it may perhaps be doubted whether an instrument written or printed on any substance capable of receiving and retaining legible characters, would not have equal validity. The instrument must necessarily be written, printed or engraved, Blackstone suggests7 that a signature of the party whose deed it is should also be added. Doubtless this is unsual and desirable, but it certainly was not requisite in the early law, and it probably is not requisite now.8 The instrument must contain a promise sufficiently definite in its terms for enforcement. A seal must be attached and the instrument must be delivered. It should be observed, however, that if parties intend to execute a sealed instrument or to execute effectively an instrument which the law requires to be sealed, but fail to attach a seal to the instrument which is signed and delivered, equity will now afford relief to rectify the mistake, and to this end may treat the instrument as if it had been sealed.9