The skin on the pads of human fingers is covered in raised ridges that form patterns known as fingerprints. The scientific study of these dermal ridges is called dermatoglyphics, confirming that these patterns remain unchanged throughout a person’s life. For classification, friction ridge patterns are sorted into three fundamental groups: the Arch, the Whorl, and the Loop. The Loop pattern, characterized by ridges that enter and exit on the same side of the print, is the most frequently encountered type across the human population.
The Anatomy of a Fingerprint Loop
A pattern is categorized as a Loop if it possesses specific structural characteristics. The defining feature is a series of ridges that flow in from one side of the print, recurve sharply, and then flow back out toward the same side. This recurving action must encompass the two focal points required for a true Loop pattern.
The first focal point is the core, defined as the approximate center of the pattern, located on or within the innermost looping ridge. The second focal point is the delta, a triangular ridge area where the ridges diverge to flow around the pattern. A loop must have exactly one core and one delta, and at least one ridge must cross an imaginary line drawn between these two points. The relationship between these two features allows analysts to perform a ridge count, a factor in individualizing a print.
Identifying the Ulnar Loop
The Loop pattern that opens toward the little finger is classified as the Ulnar Loop. This designation is derived from the ulna bone, which is the larger of the two bones in the forearm. In an Ulnar Loop, the friction ridges flow toward the pinky side of the hand, regardless of whether the print originated from the left or right hand.
The Ulnar Loop is the most prevalent fingerprint pattern, accounting for approximately 60 to 65 percent of all patterns found on human fingers. To correctly identify this pattern, an analyst must know which hand the print belongs to, since classification is based on anatomical directionality relative to the body’s structure. For example, on a right hand, the ridges flow toward the right side, which is the little finger side. Conversely, on a left hand, the Ulnar Loop flows toward the left side, which is the little finger side of that hand.
Differentiating Radial Loops
The contrasting pattern to the Ulnar Loop is the Radial Loop, which directs its ridges toward the thumb side of the hand. This name comes from the radius bone, the forearm bone situated on the same side as the thumb. The Radial Loop’s classification is determined by the specific hand and finger from which the print was taken.
Radial Loops are less common than Ulnar Loops, appearing on only about 5 percent of all fingerprints. This pattern is seen most often on the index finger, though its frequency is much lower on other digits and is virtually absent from the little fingers. The distinction is important because the classification depends entirely on the hand: a loop flowing toward the right on a right index finger is Ulnar, but the same flow on a left index finger is Radial.
Practical Applications of Loop Classification
The classification of a loop as ulnar or radial is central to the forensic science of dactyloscopy, or fingerprint identification. This classification allows large collections of prints to be systematically organized and searched, which would be impossible if every print had to be compared individually. The directionality of the loop is used to place a ten-print card into a specific category within historical systems like the Henry Classification System.
In the Henry Classification System, a Radial Loop is assigned a capital ‘R’ for the index fingers, while Ulnar Loops are indicated by a simple diagonal line due to their high frequency. While modern Automated Fingerprint Identification Systems (AFIS) use advanced algorithms, the initial pattern grouping remains an important filter for narrowing down potential matches from vast databases. The ridge count across the loop, which starts at the delta and ends at the core, provides a numerical value used for identification within the broader pattern category.