All instruments are legal documents but all legal documents are not instruments.

According to Black's Law Dictionary an instrument is a written document; a formal or legal document in writing, such as a contract, agreements, deed, will, bond, or lease. Under Section 2(b) of the Notaries Act, 1952, and Section 2(14) of the Indian Stamp Act, 1899, the word "instrument" includes every document by which any right or liability is, or purports to be, created, transferred, modified, limited, extended, suspended, extinguished or recorded. In the context of the General Clauses Act, it has to be understood as including reference to a formal legal writing like an order made under constitutional or statutory authority. Whereas legal documents includes contract, agreements, deed etc. which need not contain matters presumed or implied by law. It is better in such an agreement to specify even such matters and all other matters so as to make it a complete code, embodying the rights and duties of each party. Thus all instruments are legal documents but all legal documents are not instruments.