Forensic evidence can be used for the solving of a variety of crimes and for investigations. Its usefulness originates from Locard�s Principle, which states that "every contact leaves a trace". For example, if someone brushes past an item of furniture, fibres from his clothing attach themselves to the furniture, and conversely evidence of the furniture can be found on the person�s clothing. From the presence, nature and abundance of this trace, scientists can infer many things about the person who left it. It can help in the detection of crime, and the prosecution of a criminal.

This principle is not completely reliable when used on its own - there is another principle which must also be taken account of when examining trace evidence. This is known as the Individuality Principle, where no two objects are identical. It is often the case that two objects cannot be told apart, but they are not identical. If the two objects are distinguishable, it is obvious they are not from the same source. However, if they are indistinguishable, they need to be examined in more detail to determine whether they are from the same source.

These two principles if applied together are invaluable to the forensic scientist and the prosecution. If it can be proved that a fingerprint from the scene of a crime is indistinguishable from a fingerprint taken from a suspect, a positive piece of evidence may be presented by the prosecution to the court. In the same way, a bite mark on the body of the victim can be analysed. If the mark is shown to sufficiently match the bite of the suspect, it could be successfully proven that it was the suspect who bit the victim. Using these two principles to help them, forensic scientists can prove many things in a homicide case. In short, the whole sequence of events can be proven by a large amount of forensic evidence.

A commonly used source of forensic evidence is fingerprints. No two fingerprints are alike � identical twins do not even have the same fingerprints. The fingerprint of each finger is unique. Not only is it unique, but it does not change from when it was created before birth to when its owner dies. It may well encounter disfiguration, but scars and other features are obvious in the analysis of a fingerprint. It has been tried in the past to disfigure the fingers deliberately so as to obscure fingerprints. However, this can also be foiled. The fingers will still leave a mark � just one with no ridges. When a finger recovers from such disfiguration, it will still show signs of it having taken place, and so even this is fallible. To establish a match between a latent fingerprint and an inked fingerprint (that is, one found at the scene of the crime and one taken directly from the suspect), sixteen ridge characteristics have to be found that agree in both prints. This standard was decided upon in 1953, and was later upheld in 1978, when there was a call to reform on the standard. If there are any fewer than this amount of matching characteristics, the match will not be upheld.

Feet also leave behind a trace available for forensic analysis. If the feet are bare, they may leave a footprint just as unique as the fingerprint, while if they are clad, they may leave behind distinctive footprints, with the tread clearly visible and identifiable. This information may be less useful to the prosecution, as shoes are mass-produced and are all very similar. However, some useful information that a footprint can show is the dimensions of its owner. If the footprint is small, it is unlikely that the perpetrator is going to be six feet tall. A footprint will also show if the criminal has a distinctive walk, as the pressure on different parts of the foot will be visible.

One way of hiding fingerprints is to prevent the hazard of leaving fingerprints by wearing gloves. This also leaves remains at the scene of the crime however. Fibre remains are used less often than fingerprints or DNA evidence, but nevertheless they are still useful. The reason that they are not so often used is that such fibres are man made and mass-produced, and so there is a smaller probability that two matching fibres are going to originate from the same source. Only with a large amount of other circumstantial evidence will such fibre evidence be held as good enough in court. Another problem which can be encountered in the same subject is a material that does not leave much of a trace, for example leather. It is then very difficult to track down the accused, as he has left little or no physical trace.

Other sources of evidence connected with a homicide include weaponry. Different weapons leave different trace evidence. Such evidence can be found at the scene of the homicide, in the possession of the accused, or during a post mortem. They range from wound patterns found in the flesh and muscles of the victim to discarded bullets and the actual weapon. Where the actual offending weapon is not found, a post mortem can reveal to the pathologist with what instrument someone was killed. A bullet could be found in the body, making it quite obvious that a firearm of some sort was involved; strangulation may be indicated by the presence of bruising around the neck or the hyoid bone being broken; the patterns around stab wounds can often show what sort of object was used to stab the victim. They may also show the height of the offender, from the angle and positioning of the stab wound, and it may also be able to show whether the offender is right or left handed. Stab wounds can show different things according to where they can be found on the victim�s body. If, for example, there are wounds to the victim�s forearms, it is highly probable that the victim tried to resist and protect himself.

Other evidence connected to the crime can also be found in some cases. For example, the presence of entomological evidence can be very useful. Depending on how advanced in their development they are, the presence of maggots or flies could fairly accurately show how long the body has been dead. It can also suggest if the body has been moved since the victim was killed, and if so, what conditions it was kept in during this period and how long this period lasted. This information could be a vital clue as to the identity of the murderer.

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