What is a fingerprint?

A human fingerprint is an impression pattern left on any hard smooth surface by the ridged dermis (skin) (called friction ridges) of the fingertip. The fundamentals of fingerprint identification are permanence and individuality. In theory, significant fingerprint characteristics are determined by environment in the 3rd and 4th lunar months of gestation, and do not change. In practice, scars and warts can modify fingerprints somewhat. Popularly, no two individuals, including identical twins, are thought to have the same fingerprints.

In 1892, Sir Francis Galton found that the typical fingerprint has 35-50 identification points ("minutiae") and the odds of two being alike are 1 in 64 billion. With a 2006 world population of 6.5 billion individuals (and thus 65 billion fingertips), it would be impossible for all of them to be different. In practice, however, identifications are commonly made from as 8 to 12 minutiae. Using only a fifth of the minutiae would reduce the potential number of different fingerprints.

Ordinary fingerprints made of sweat, sebum, dirt, and skin contaminants are called latent fingerprints, to distinguish them from the comparison imprints deliberately taken with inked fingers on "fingerprint cards".