The Dichotomous Key

If it has these attributes
then it is this.  
Otherwise it is not this.
		
& if it is not this, but has 
these other attributes 
then it is that.  
Otherwise it is not that.
		
If it is neither this nor 
that, but possesses another
set of characteristics 
then…..

& so on 
down the line, eliminating
the alternatives by
counting in turn the
wings, legs, body 
segments, etc. 

so that you 
can start off
with the spider
& end up
with the fly.