A matter or thing (such as a signature) may be authenticated by having the trier of fact compare that matter or thing with another matter or thing -- a "specimen" or "exemplar" -- that has been authenticated.

Fed. R. Evid. 803(d)(2)(E) ("a statement by a coconspirator of a party during the course and in furtherance of the conspiracy").A statement is exempted from the prohibition against hearsay by the co-conspirator hearsay exemption only if the evidence submitted to the trial court shows by a preponderance of the evidence that the maker of the statement and the party against whom the statement is offered were probably members of a conspiracy; the statement sought to be admitted must be shown to be, more probably than not, the statement of a co-conspirator of the party against whom the statement is offered.

Cage v. Lousiana, 498 U.S. 39, 41 (1990) ("It is plain to us that the words ?substantial? and ?grave,? as they are commonly understood, suggest a higher degree of doubt than is required for acquittal under the reasonable doubt standard. When those statements are then considered with the reference to...A jury instruction that the jury may acquit only if it has a grave and substantial doubt about guilt is unconstitutional.

Upjohn Co. v. United States, 449 U.S. 383, 389 (1981) ("Federal Rule of Evidence 501 provides that 'the privilege of a witness . . . shall be governed by the principles of the common law as they may be interpreted by the courts of the United States in light of reason and experience.' The attorney-client...There is an attorney-client privilege.