Humans have characteristically ridged skin on their fingertips, palms, and soles. This roughened skin makes it easier to grip things and, up close, it appears as patterns of tiny ridges and furrows. The fingertips, palms, and soles can sometimes create a transfer of these patterns when they come into contact with surfaces and objects. The most important of these transfers are fingerprints, made when the tips of the fingers and thumbs make impressions. Fingerprints have long been used for forensic identification purposes thanks to features within their patterns called ridge characteristics or minutiae.
All fingerprints fall into one of three basic overall patterns, the arch, the loop, and the whorl. However, the ridges themselves form a wide variety of patterns within these basic three types. Fingerprint experts describe various ridge characteristics. For example, ridge endings refer to an abrupt cessation of ridge. A bifurcation occurs when a ridge splits into two. A dot is a very small segment of ridge. There are also combinations of ridge characteristics, such as the island that is two bifurcations together. When a control fingerprint, either taken from a suspect or obtained from a database, is compared with one from the scene of a crime, the investigator will look at the ridge characteristics.
The control and the sample fingerprint are placed in the same orientation and a search is made for ridge characteristics that match. Each person has a unique pattern of ridge characteristics and it is this mark of identity for which the investigator must search. The number of ridge characteristics that must match to allow identification remains debatable. For many decades, investigators had to match a minimum of 12 ridge characteristics in a control and sample fingerprint to be able to say they came from the same finger. Now, however, it is accepted that having a fixed minimum is not appropriate in all cases and it is best left to the experience of the investigator to make the decision on identification. Of course, he or she should be prepared to defend this decision in court.
see also Fingerprint; Fingerprint analysis (famous cases); Latent fingerprint.