Section 4(b) in The Official Languages (Use For Official Purposes Of The Union) Rules, 1976
(b)between one Ministry or Department of the Central Government and attached or subordinate offices situated in Region "A" shall be in Hindi and in such proportion as the Central Government may, having regard to the number of persons having a working knowledge of Hindi in such offices, the facilities for sending communications in Hindi and matters incidental thereto, determine from time to time;
