The Locard exchange principle, also known as Locard's theory, was postulated by 20th century forensic scientist Edmond Locard

Edmond Locard

Dr. Edmond Locard 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"...

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Locard was the director of the very first crime laboratory in existence, located in Lyon

Lyon

Lyon , is a city in east-central France in the Rhône-Alpes region, situated between Paris and Marseille. Lyon is located at from Paris, from Marseille, from Geneva, from Turin, and from Barcelona. The residents of the city are called Lyonnais....

, France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan 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 idiom

Idiom

Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

"every contact leaves a trace".

"Everywhere you go, you take your behind." is the premise behind Locard's theory.

Essentially, Locard's principle is applied to crime scenes in which the perpetrator(s)