Section 16(4) in The Indian Post Office Rules, 1933
(4)The minimum size of "Business Reply Envelopes" shall conform to the minimum, size prescribed for covers of letters. Business Reply Letter Cards shall conform in size, quality and other conditions to those prescribed for letter cards of private manufacture. The "Business Reply Cards" shall conform in size and quality with conditions prescribed for postcards of private manufacture. The address side of each such card shall be reserved fully for the particulars given in sub-rule (5) and the other side may be used by the sender for the purpose of written communication, or may be otherwise disposed of, provided that nothing shall be attached to any side of such card except :-(a)in the case of address side, a Business Reply referred to in sub-rule (5); and(b)in the case of the other side -(i)a gummed label not exceeding 2 inches (5cms) in length and ¾ (2cms) in width, completely adherent to the card and bearing the name and address of the sender of the card; and(ii)engravings, illustrations, drawings and photographs on very thin paper and completely adherent to the card.
