Explain the meaning of ambiguous instrument under the Negotiable Instruments Act, 1881.

According to Section 17 of the Negotiable Instruments Act, 1881 an instrument, which in form is such that it may either be treated by the holder as a bill or as a note, is an ambiguous instrument. Section 5(2) of the English Bills of Exchange Act provides that where in a bill, the drawer and the drawee are the same person or where the drawee is a fictitious person or a person incompetent to contract, the holder may treat the instrument, at his option. either as a bill of exchange or as a promissory note. Bill drawn to or to the order of the drawee or by an agent on his principal, or by one branch of a bank on another or by the direction of a company or their cashier are also ambiguous instruments. A promissory note addressed to a third person may be treated as a bill by such person by accepting it, while a bill not addressed to any one may be treated as a note. But where the drawer and payee are the same, e.g., where A draws a bill payable to A's order, it is not an ambiguous instrument and cannot be treated as a promissory note. Once an instrument has been treated either as a bill or as a note, it cannot be treated differently afterwards.