CLASSIFYING FINGERPRINTS
The patterning and permanency of friction ridges allow for their classification. As discussed earlier, the fact that fingerprints could be systematically sorted and catalogued was a main reason for their widespread adoption among government agencies. The history of the classification of fingerprints is central to the understanding of the widespread adoption of the technique; see “History: Classifying Fingerprints” for more information. It is important to keep in mind that it is the general patterns, not the individualizing elements, that allow for this organization.
Today, all fingerprints are divided into three classes: loops, arches, and whorls. Loops have one or more ridges entering from one side of the print, curving back on themselves, and exiting the fingertip on the same side, as shown in Figure 1. If the loop enters and exits on the side of the finger toward the little finger, it is called an ulnar loop; if the loop enters and exits on the side toward the thumb, it is termed a radial loop. All loops are surrounded by two diverging ridges called type lines; the point of divergence is called a delta because of its resemblance to a river delta and the Greek letter ∆ (delta). The central portion of the loop is called the core. Arches are the rarest of the three main classes of patterns. Arches are either plain arch (see Figure 2), with ridges entering one side of the finger, gradually rising to a rounded peak, and exiting the other side, or tented arch, which are arches with a pronounced, sharp peak. A pattern that resembles a loop but lacks one of the required traits to be classified as a loop can also be designated as a tented arch. Arches do not have type lines, cores, or deltas. Whorls are subdivided into plain whorl (see Figure 3), central pocket loop, double loop, and accidental. All whorls have type lines and at least two deltas. Central pocket loops and plain whorls have a minimum of one ridge that is continuous around the pattern, but this ridge does not necessarily have to be in the shape of a circle; it can be an oval, ellipse, or even a spiral. Plain whorls are located between the two deltas of the whorl pattern and central pocket loops are not. This difference can be easily determined by drawing a line equidistant between the two deltas: If the line touches the circular core, then the whorl is a plain whorl; if not, it is a central pocket loop (see Figure 4).
FIGURE 1 Loops have one or more ridges entering and exiting from the same side of the print, looping back on itself in the middle. Ulnar loops exit and enter the side of the finger toward the little finger, whereas radial loops are on the side toward the thumb.
FIGURE 2 Arches enter one side of the fingertip, peak, and then exit the opposite side. Arches are either plain or tented.
FIGURE 3 All whorls have type lines and at least two deltas. Whorls are classified as plain, central pocket loops, double loops, and accidentals.
FIGURE 4 To determine whether a print is a plain whorl or a central pocket loop, draw a line between the two deltas. If the line touches the core, it’s a plain whorl; if not, then the print is a central pocket loop.
FIGURE 5 Double loops have two loops that swirl around each other; accidentals combine two or more patterns.
A double loop is made up of two loops that swirl around each other (see Figure 19.10). Finally, an accidental is a pattern that combines two or more patterns (excluding the plain arch) and/or does not clearly meet the criteria for any of the other patterns. The relative appearance of loops overall is 60–65%; whorls, 30–35% and arches, 5%.
