The Locard exchange principle, also known as Locard's theory, was postulated by 20th century forensic scientist Edmond Locard.
Locard was the director of the very first crime laboratory in existence, located in Lyon, France. Locard's exchange principle states that "with contact between two items, there will be an exchange" (Thornton, 1927) and is known most commonly as the idiom "Every contact leaves a trace."
"Everywhere you go, you take something with you, and you leave something behind." is the premise behind Locard's theory.
Essentially, Locard's principle is applied to crime scenes in which the perpetrator(s) of a crime comes into contact with the scene, so the perpetrator(s) will both bring something into the scene and leave with something from the scene.
| “ | Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value. | ” |
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—Paul L. Kirk. 1953. Crime investigation: physical evidence and the police laboratory. Interscience Publishers, Inc.: New York. |
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Fragmentary or trace evidence is any type of material left at—or taken from—a crime scene, or the result of contact between two surfaces, such as shoes and the floor covering or soil, or fibers from where someone sat on an upholstered chair.
When a crime is committed, fragmentary (or trace) evidence needs to be collected from the scene. A team of specialized police technicians go to the scene of the crime and seal it off. They both record video and take photographs of the crime scene, victim (if there is one) and items of evidence. If necessary, they undertake a firearms and ballistics examination. They check for shoe and tire mark impressions, examine any vehicles and check for fingerprints. The following are some of illustrations of the usefulness of forensic science laboratory in criminal investigations....1.personal identity,,,,2.blood,,,,,,3.semen,,,,,,4.firearms,,,,5.fingerprints,,,,,,6.hair,,,,7.fibers,,,,,8.poisons,,,,,9.clothes,,,,,10.weapons$tools,,,,,11.glass,,,,12.wood,,,,13.metalsetc...
"Locard's Exchange" is the title of a 2004 episode of the television crime drama Crossing Jordan.
In the TV show CSI, Season 9, Episode 5, near the end of the episode, Nick Stokes says "According to Locard's Theory, we are missing anything".