**3. Definition of tool marks**

Tools are mostly directly related to object markings, because at the time of tool production, such designs or irregularities are imprinted on their surface, so it is implied that these patterns and variations might be part of the identification features of a particular object; for example, marking bullets can lead to a particular firearm. Furthermore, these substantially different types and irregularities of the instrument can be visually identified and compared using forensic techniques [12].

The term "tool mark" is defined in a number of ways. A widely accepted AFTE definition defines tool mark as "If any object or instrument reaches the surface with enough force to allow its signature design to be indented, this form of marking is referred to as a tool mark.". In another definition [6], it is stated that "An instrument that is considered to be sufficiently stronger from two objects acquires comprehensive force when it comes into contact with each other, which leads to the softer one being marked.".

Biasotti and Murdock [13] state that "When two objects begin to interact, the extremely hard object will stamp the surface of the softer object. The relative hardness of the two artifacts, the pressures and motions, and the appearance of the microscopic discrepancies on the object are all factors influencing the character of the generated toolmarks." It is necessary to establish the correlation between a tool mark and the tool that produced it in criminal investigations such as burglaries. For instance, if a burglar chooses wooden or metal bars to force entrance into a home, the marks left by the tool on the doorway are strong evidence of the involvement of that tool for that legitimate purpose at the scene of the crime. If the tool is linked with, or close to, a suspect, it enables for the identification of a link between the accused person and the incidence of the crime.
