A cheque in not an stamped instrument, hence it is not covered by Section 20.
Section 20 of the Act makes it clear that there can be no material alteration of a cheque leaf only, for the reasons, that it was subsequently filled up. But at the same time, it cannot be said that whenever a signed blank cheque leaf is given, it gives authority to the holder to fill up the same according to his whims and fancies. Filling up of a signed blank cheque leaf may not attract section 87 of the N.I. Act for, there was no insertion, interlineation, erasure, alteration etc., because there was no completed negotiable instrument within the meaning of sections 5, 6 and 13 of the N.I. Act. Therefore, neither section 20 nor section 87 applies to a blank signed cheque leaf. If so, the question must turn round to the actual execution of the instrument. With regard to the instruments other than a cheque, an implied authority is given to the holder at the time of entrusting it to fill up the same. There may be instances where an implied authority is given to the person, at the time of entrusting a signed blank cheque containing the signature of the drawer of the cheque, to fill the Columns therein.