Locard's exchange principle

The Locard Exchange Principle was created by 20th Century by criminologust Edmund 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."

Essentially Locard's principle is applied to crime scenes in which the perpetrator(s) of a crime comes into conact with the scene, so he/she will both bring something into the scene and leave with something from the scene. Every contact leaves a trace.