Intro to the Federal Rules of Evidence - 24

B. AUTHENTICATION
1. Authentication
All real and demonstrative evidence must be authenticated before it is admitted. That is, it must be shown to be genuine. This means that the object must be established to be what its proponent claims it to be (FRE 901(a)).

a. Real evidence
It the object is real evidence, authentication usually means showing that the object is the object that was involved in the underlying event.
For example, the actual knife used in the stabbing; a contract signed by P & D; a tape recording a conversation in which D tried to bribe a public official .

b. Demonstrative evidence
If the evidence is demonstrative, authentication usually means showing that the object fairly represents or illustrates what it is claimed to represent or illustrate.
For example, proof that a diagram offered in evidence really shows the position of the parties and witnesses at the time of the murder.