law: Indian Evidence Act, 1872 
  47. Opinion as to handwriting, when relevant 
  When the Court has to
  
  form an opinion as to the person by whom any document was written or signed,
  
  the opinion of any person acquainted with the handwriting of the person by whom
  
  it is supposed to be written or signed that it was or was not written or signed
  
  by that person, is a relevant fact.
  Illustration
  The question is,
  
  whether a given letter is in the handwriting of A, merchant in London.
  B is a merchant in
  
  Calcutta, who has written letters addressed to A and received letters
  
  purporting to be written by him. C is B’s clerk, whose duty it was to examine
  
  and file B’s correspondence. D is B’s broker, to whom B habitually submitted
  
  the letters purporting to be written by A for the purpose of advising with him
  
  thereon.
  The opinions of B, C
  
  and D on the question whether the letter is in the handwriting of A are
  
  relevant, though neither B, C and D ever saw A write.