Fingerprint. Coloured scanning electron micrograph (SEM) of part of a human fingerprint, showing details of skin ridges in the outer epidermis. The small circular apertures on the ridges are the openings of sweat glands. Epidermal ridges occur on the soles and palms, and they form a distinct pattern. Each fingerprint pattern 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 three.