Five simple geometric elements constitute all patterns observable by dermatoscopy. These basic elements are lines, pseudopods, circles, clods, and dots (Fig. 1).

Lines can be arranged to form five different patterns: reticular, branched, parallel, radial, and curved (Fig. 2). Apart from lines, basic patterns may be created by the other four fundamental elements: pseudopods, circles, clods, and dots (Fig. 3). Last, there is one pattern characterized by the absence of basic elements, that being structureless (Fig. 3).

Variations of basic patterns come into being by variations in the morphologic appearance of basic elements or by variations in distribution of those elements. Examples of the former are lines reticular that can be thin or thick (Fig. 4), and examples of the latter are parallel lines situated in furrows or in ridges of glabrous skin or crossing furrows and ridges (Fig. 5). Pigmented skin lesions may consist of a single pattern or of two or more patterns. Two or more patterns can be combined in a symmetric or asymmetric fashion (Fig. 6).

The different colors visualizable by dermatoscopy come into being by virtue of presence of keratin, melanin, blood (including serum in crusts), foreign material, or combinations of those constituents. The color of melanin as discerned by dermatoscopy depends where, precisely, it is situated in the epidermis or dermis (Fig. 7).

Clues as employed here refer to elements, patterns (including those formed by vasculature), colors, or combinations of them that are characteristic, although not diagnostic unequivocally, for a specific pigmented lesion in the skin. For example: Thick dark brown or black reticular lines are characteristic for "ink-spot lentigo" (Fig. 8). Another example is white dots as clue to seborrheic keratosis. White dots in conjunction with curved lines are an even more powerful clue, they being indicative of a diagnosis of seborrheic keratosis. However, a clue also may steer a dermatoscopist in the wrong direction. White dots may be found not only in seborrheic keratosis, but rarely in lesions as different as melanoma, Clark nevus, all types of congenital nevi, and basal-cell carcinoma.

All morphologic structures seen by dermatoscopy can be described by a constellation of basic patterns formed of elements and colors. There is no need for metaphoric terms and no such terms are used in the schema advocated here. For example: (1) "Blue gray ovoid nest" means blue or gray clods (2) "lacunae or saccules" means red clods (3) "radial streaming" means radial lines at the periphery (4) "spoke-wheel areas" means radial lines converging to a central dot or clod, and (5) "a fingerprint-like pattern" means curved lines, etc. There is no need ever for metaphoric terms and opaque language.

The algorithm for diagnosis is based on interpretation of patterns, colors, and clues, and directs the observer to a diagnosis with specificity. It is a procedure that uses multiple steps in logical sequence in which every step appears in the same order for any type of lesion. The first step is to differentiate between pigmented lesions with a single pattern and ones with two or more patterns. Lesions with a single pattern are characterized further by their color(s). If a specific diagnosis cannot be reached at this juncture, one must then search for clues. Coming to a diagnosis for lesions with two or more patterns is not different fundamentally from the method used to make a diagnosis when there is but a single pattern. To reach a specific diagnosis, one must determine whether the combination of patterns is symmetric or asymmetric. After that step has been taken, one must consider the colors and, last, the clues. In order to demonstrate how the algorithm is applied "in real life," one example of a pigmented lesion is pictured now (Fig. 9 and 10). The algorithmic approach to this lesion is pictured in Figures 11–13. Only one pattern is present, it being reticular. There are two different colors: light brown at the periphery and dark brown in the center. This combination of pattern reticular and colors in distribution symmetrical leads to the diagnosis of Clark nevus (Fig. 14), which is confirmed by examination histopathologically (Figs. 15–20).

The algorithm for the entirety of pigmented lesions in the skin, including non-melanocytic lesions, will be available soon on derm101.com.