# How to Make a Dichotomous Key: 10 Steps (with Images)

The dichotomous key is a way to identify specimens based on contrasting statements, often regarding physical characteristics. By drawing a series of contrasts, you can delimit the specimen until you can identify it correctly. Keys are widely used in sciences such as biology and geology. To make yours, you first choose the characteristics that you will use to contrast the specimens and then formulate them as a series of statements or questions that you can use to delimit them.

## Steps

### Part 1 of 3: Analyzing the specimens

#### Step 1. List the characteristics of the specimens

Start by idealizing which ones you want to identify and “key”. Note the defining characteristics of the animals under analysis and list them.

• Example: When trying to create a key for a series of animals, you will probably notice that some have feathers, some swim, some walk, etc.
• When differentiating a group of large cats in pictures, you may notice that some are brown, some are black, some have stripes, some dots, and some may have long tails, some may have shorter tails, and so on.

#### Step 2. Look for exclusion principles

The dichotomous key uses the process of elimination, so it is important to note characteristics that can be used to differentiate between the animals you are analyzing. Example: in case some analyzed specimens have feathers, but others have fur, “feathers” will be a good differentiating characteristic.

### However, being animals or birds, “warm blood” would not be a good differentiating factor, as it is common to all of them and will not help to divide the specimens into smaller groups

#### Step 3. Define the most general characteristics

Create a key based on the specific differences in ascending order, that is, you will have to sort the characteristics from general to specific. Thus, the specimens will be divided into even smaller groups. For example:

• Perhaps some cats have dark fur and others lighter. In addition, you also notice that they all have short fur, some with long tails and some without a tail.
• You could start the key with a question or statement about hair color. Afterwards, it will not be necessary to ask about coat size, as all examples have short fur. The next question will be about the size of the tail, since you don't see tails in certain species of cats and, therefore, this is a less generalized characteristic.

### Part 2 of 3: Creating your own dichotomous key

#### Step 1. Formulate several differentiating steps

It's up to you whether you're going to use questions or statements, although asking questions is more intuitive. In both cases, each question or statement should divide the analyzed specimens into just two groups.

• “The cat has solid-colored fur” or “The cat has patterned fur” are statements that can be used to divide them into two distinct groups.
• "Does the cat have solid color fur?" is a question that divides the specimens also into two groups: answer “Yes” places the cats in the group that has solid colored fur; Answer “No”, puts the cats in the group that has patterned fur.

#### Step 2. Divide them into two groups

Here the first differentiation will take place, and it should be based on the most general aspect of the specimens. So, review the list of physical characteristics you made. Name the first two groups A and B.

### Example: when noticing that all specimens under analysis have feathers or scales, these two subdivisions could be A and B. You can start the key with the question “Does the animal have feathers?”

#### Step 3. Subdivide each of the first two groups into two more groups (subgroups)

Group A and B will both be divided into more specific groups (C and D), based on the following differentiating feature.

• Example: You may notice that some group A animals swim and some don't. This differentiation may form level C and D of group A.
• Likewise, some animals in group B have legs and some do not. Here, levels C and D of group B will be formed.

#### Step 4. Continue to break down the groups and formulate questions or statements of increasing specificity based on the physical characteristics you have identified

Find characteristics that can divide the specimens, as needed, into groups E-F, G-H, and so on. At some point, there will come a point where you will have questions asking just to differentiate the two specimens. At this point, your dichotomous key will be complete.

• Some specimens will be differentiated before the end of the key as you fill in the distinct characteristics. Example: you are analyzing some birds and reptiles. When dividing them into these groups, just subdivide the birds.
• Two of the birds swim, but one of them doesn't. The only one that is terrestrial will be identified as such, but you will have to differentiate the ones that swim in more detail.
• In that case, you find that one of them swims in oceans and the other doesn't. This feature will allow you to identify them more accurately (examples: the seagull and the duck).

### Part 3 of 3: Completing the Dichotomous Key

#### Step 1. Draw it in graph form

A dichotomous key can be made text-only on a basis with simple question sequences. However, bringing visuality can give the material another tone. You can, for example, create a “tree-shaped diagram”, where each consecutive level of differentiation forms another branch of the tree.

### You can also try to organize it in flowchart form. Example: in one of the text boxes, put a question like “Does the cat have dark fur?”, an arrow written “Yes” going in one direction and another one written “No” in the other direction. The arrowheads lead to new text boxes where you will ask the next questions

#### Step 2. Test it

When you have all the information written down and organized, test the key using any specimen to see if it works. Example: let's say you have a dichotomous key that helps identify multiple animals. Take a specimen and answer the questions until the key leads you to make the identification through elimination:

• P: "Does the animal have feathers?" A: "No" (he has scales, so he's a reptile).
• P: "Does the reptile have legs?" A: “No” (it is a snake, a cobra or a python, depending on the species).
• P: "Does the snake have a hood?" A: "No" (so it's not a cobra)
• Your specimen is a python snake.

#### Step 3. Fix the key if there is any problem

It may not be working properly and needs adjustment. Example: you may not have ordered the questions in such a gradual and specific way, that is, you will need to reorganize them, as well as perhaps the key cannot divide the specimens in a more logical way. Complete a rephrasing of the questions.

• Example: "Does the cat have a solid or striped coat?" is not a useful question for a dichotomous key. This question may even differentiate solid-colored and striped cats from those with spots, but it won't serve as a category.
• Prefer to ask a question first that you want to know about those with solid colored fur versus those with patterned fur, then go on with another level of questions (“Does the cat have dark fur?” and “Does the cat have stripes?”).