Section 46C(1) in The Indian Post Office Rules, 1933
(1)No article shall be transmitted by post in any envelope with an open (cutout) panel which does not conform to the following conditions, namely:-(a)the open (cut-out) panel must not exceed 10cm. in length by 3.5 cm. in breadth;(b)the panel must extend parallel to the length to the envelope.(c)a minimum space of 1.5 cm. must be left above the panel for the postage stamp and the date-stamp and the date-stamp impression;(d)no writing or printing other than the address may be visible through the panel and the address must appear through the panel in such a manner as can be read easily;(e)the enclosure must be so folded that it cannot move about in the envelope and thus cause the address to be hidden;(f)a minimum space of at least 1.5 cm must be left between the sides and base of the cut-out panel and the respective edges of the envelope.
