Fingerprints. Man's index finger producing an inked print, next to prints of his other fingers. A fingerprint shows details of skin ridges, which form a distinct pattern that is unique to an individual. Even identical twins have different fingerprints. The patterns are classified into four main types: loops, whorls, arches, and combinations of these. These differences are used by forensic scientists when detecting crime. When a finger touches most surfaces an impression of the fingerprint is left, this can be recorded and later matched against inked fingerprints on file.