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Start by considering the specimens you are trying to identify and insert into a dichotomous key. Note characteristics that define the things you are looking at, and start listing them out.   If you are trying to create a dichotomous key for a series of animals, you might see that some have feathers, some swim, some walk on legs, etc. For example, if you are trying to differentiate a set of big cats, you might note that some are brown, some are black, some have stripes, some have spots, some have long tails, some have short tails, and so on. A dichotomous key works by the process of elimination, so you want to note characteristics that can be used to differentiate the things you are examining. For example, if some of the specimens you are looking at have feathers but others have fur, then “feathers” is a good distinguishing characteristics.  However, a trait all of the animals share is not a good distinguishing factor. For example, since all big cats are warm-blooded, you wouldn't want to use that trait on your dichotomous key. You want to create a dichotomous key based on increasingly specific differentiations, so you’ll have to order the characteristics of your specimens from general to specific. This will help divide your specimens ever-smaller groups. For instance:   When making your dichotomous key for big cats, you may find that some of the cats you are analyzing have dark fur, and some have light fur. You may also see that all of them have short hair. Finally, you see that some of them have long tails, but some of them have no tails at all. You would start your key with a question/statement about fur color. You wouldn’t need to ask a question about fur length, since all of the examples have short fur. You would follow up with a question about tail length, since tails are not common to all of the cats, and therefore are a less general characteristic.
List the characteristics of your specimens. Look for principles of exclusion. Determine the most general characteristics.