Intro to the Federal Rules of Evidence - 25

2. Methods of authentication
a. Real evidence
i) Readily or uniquely identifiable
if the item has a unique, one-of-a-kind characteristic, this method can be used. The sponsoring witness merely testifies that the object he originally saw has a specific unique character, and that the item shown to him in court bears that same unique identifier.

Example: D is charged with stabbing V to death. W is a police officer who inspected the crime scene shortly after the killing occurred. After W testifies about beginning the inspection of the crime scene, the following testimony would authenticate the knife as the murder weapon.
PROSECUTOR: What did you do during the inspection?
WITNESS: I looked for any evidence of what had killed the deceased.
PROSECUTOR: what did you find?
WITNESS: I found a knife next to the body, which was a pearl-handled switchblade about ten inches long, coated with a sticky red substance.
PROSECUTOR: what did you do then?
WITNESS: in the case of a comparatively inexpensive object like this one, I mark it for identification by scratching my initials and the date into the handle.
PROSECUTOR: I show you People’s Exhibit No. 1 for identification. What is it?
WITNESS: It’s the knife I found.
PROSECUTOR: How can you tell that that’s what it is?
WITNESS: As I said, I marked my initials and the date on the handle. They’re right there.
PROSECUTOR: Has the Exhibit changed since you found it?
WITNESS: No, it seems to be in the same condition I found it.
PROSECUTOR: Your honor, I now offer People’s Exhibit No. 1 for identification into evidence as People’s Exhibit No. 1.
COURT: It will be received.

ii) Chain of custody
This method is used when one of the elements for ready identifiability is absent.
A key reason for the elaborate chain-of-custody method is to prevent tampering with evidence.