The Writers’ Police Academy is by far the best conference I’ve ever attended. While there, I could hardly wait to share all the things I learned with you. In today’s post, let’s look at fingerprints. My instructor was an incredible teacher, but it’s a difficult field to grasp in a short period of time. So when I got home I delved deeper into fingerprints in preparation of this post. Hopefully, I can save you time if you choose to use this information in your books. That’s my goal, anyway. Here we go…

Fingerprints

A fingerprint is a pattern of friction ridge details that are comprised of ridges and valleys. A ridge is a high point. A valley is a depression or low point. Friction ridges are also found on our palms, feet, and toes. The pattern is the unique characteristics of the ridges and valleys that make up the print. It is defined by the spatial relationship of lines with each other, their beginning and terminating points, and the unique pattern they make. The genes from our parents determine the general characteristics of the pattern.

Sir Francis Galton was the first person to classify fingerprints into different types based on the three basic features: Loops, Arches, and Whorls. We’ll delve deeper into Loops, Arches, and Whorls in a minute. Fingerprints form on a person before birth and remain unchanged until the body decomposes after death. The only exception would be an injury to the print. For example, if someone sliced their fingertip with a knife. But then, their fingerprint would be even more distinguishable because of the scar.

Before anyone asks, twins do not have identical fingerprints. Our prints are as unique as snowflakes falling from a winter sky.

In order to learn how to identify fingerprints, we need to know what we’re looking at and where to find the pattern. If we examine a fingerprint, we need to study the Pattern Area — the place where we can classify into type. The Pattern Area contains the Core, Delta(s), if any, and Ridges. I’m capitalizing for clarity purposes only. If you use this information in your book, these words are not normally capitalized.

In the image (sorry the pics are a bit blurry), the right line shows the Core. As you can see, in a Loop the approximate center of the finger impression is the core.

On the left side of the image, we can see (from top to bottom) Type Lines, Delta, and more Type Lines.

TYPE LINES

Two innermost ridges which start parallel, diverge, and surround, or tend to surround, the Pattern Area.

DELTA

The point at, or in front of, and nearest the center of, the divergence of the Type Line. In simpler terms, a Delta is where the ridges form a triangular-shape. Can you see the tiny triangle in the lower left corner of the image above? It’s marked Delta and underlined in green. This is important because the Delta(s) determine how a fingerprint is classified.

There are three classifications of fingerprints.

• Loops
• Whorls
• Arches

If a fingerprint has one Delta, it’s classified as a loop.

If a fingerprint has two Deltas, it’s classified as a whorl.

If a fingerprint has no Deltas, it’s classified as an arch.

Still with me? Great. Moving on…

This is where it becomes more difficult. Some departments require a 12 point match to a suspect’s prints. However, in the U.S. there is no standard requirement. The match is left to the individual Fingerprint Examiner. Even after IAFIS (Integrated Automated Fingerprint Identification System) confirms a match, the Fingerprint Examiner must manually confirm the points of identification.

In England, the standard is 16 points. In France, the standard is 17 points. And in Germany, the standard is 12 points.

The image below shows the points of identification. Points are ridge characteristics, and there are as many as 150 points in the average fingerprint. Amazing, right?

Points of Identification

MINUTIAE — unique ridge patterns with small details that are used to positively match a fingerprint to a suspect. Fingerprint Examiners look at the differences between ridges, number of minutiae, and location on the impression. These factors determine the points of identification.

Obviously I can’t list all 150 points here, but here are the most common…

BIFURCATION (aka FORK)

ENDING RIDGE/OPPOSED BIFURCATION

DOT

HOOK (aka SPUR)

BRIDGE

DOUBLE BIFURCATION

PORE

Less common…

SHORT RIDGE (aka ISLAND)

ENCLOSURE (aka LAKE or EYE)

“T” JUNCTION

Rare…

RIDGE CROSSING

TRIFURCATION

ROW OF DOTS

A picture paints a thousand words. In the images below you’ll get a feel of what many of the points look like.

Delta, Whorls, and Arches

66% of the population have Loops. If the ridge lines enter from either the right or left and exit from the same place they entered, it’s classified as a Loop.

30% of the population have Whorls. Whorls look like a bullseye.

Sub-categories of Whorls

• Plain Whorl (found in 24% of the population) have one or more ridges that form a complete spiral, with two Deltas. If we draw a line between the two Deltas, at least one ridge that stems from the Pattern Area should be cut by the line.
• Central Pocket Loop Whorl (found in 2% of the population) have one or more ridges that make a complete circle, with two Deltas. If we draw a line between the Deltas, no inner pattern is cut by the line.
• Double Loop Whorl (found in 4% of the population) has two Deltas.
• Accidental Whorl (found in only 0.01 % of the population) is comprised of two Deltas and is combined with two other points.

About 5% of the population have Arches. Arches don’t contain Deltas.

Sub-categories of Arches

• Plain Arches (found in 4% of the population) enter from one side, rises in the center, and exits on the other side without forming an angle.
• Tented Arches form an angle, or may possess a characteristic of the Loop, similar to a Delta.

Types of Fingerprints

Patent fingerprints — visible prints left on a smooth surface.
Plastic fingerprints — indentations left in a soft material.
Latent fingerprints — hidden prints left by the transfer of oils or other body secretions. Latent fingerprints can be made visible by dusting with powder or via chemical reaction.

Here’s an extremely useful chart that shows the chemicals used to make Latent prints visible.

How To Examine Fingerprints

The Fingerprint Examiner will first look at the pattern type (loops, arches, or whorls). The second observation will be the line of flow, either right or left.

The third observation will be the points of identification. They’re looking for the most obvious points. What catches their eye first?

The fourth observation is to ensure the characteristics are in the same relative position. Because inking, pressure, failure to roll fingerprints nail-to-nail, and scars can all change the appearance of the characteristics.

The fifth observation is to ensure the fingerprints are in sequential order by checking the rolled impressions to the plain impressions. Note: Flat or plain impressions give a truer reading of how the ridges appear. Which is why, I’m guessing, most departments these days use live scanners to capture fingerprints rather than the ol’ roll in ink method.

I hope this helps you to write a more convincing scene. Please join me Friday when I interview Larry Brooks. I guarantee you’ve never seen this side of him…the husband who loves his wife desperately and the man behind the craft guy. You won’t want to miss this one!