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co-signatory

co-signatory, a. and n.
  (kəʊˈsɪgnətərɪ)
  [f. co- + signatory.]
  A. adj. Signing jointly with others, uniting with others in signing.

1891 Daily News 21 Nov. 5/5 The co-signatory powers to the Berlin guarantees.

  B. n. One who signs (a document, treaty, etc.) jointly with another or others; a joint signatory.

1865 Pall Mall G. 29 Sept. 4/2 It is thought that Austria..will avoid the difficulty of becoming a co-signatory of Italy. 1867 C. W. Bingham in Times 9 Nov. 10/1 He has requested me to acknowledge on behalf of ourselves and our co-signatories the receipt of your courteous reply.

Oxford English Dictionary

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