Locard's Exchange Principle

Locard's Exchange Principle

In forensic science, Locard's principle holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence.Dr. Edmond Locard (13 December 1877 – 4 May 1966) was a pioneer in forensic science who became known as the Sherlock Holmes of France. He formulated the basic principle of forensic science: "Every contact leaves a trace". This became known as Locard's exchange principle. Paul L. Kirk expressed the principle as follows:

"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."

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.