Describe Key as taxonomical aid. 

Explanation

Key is taxonomical aid used for identification of plants and animals based on the similarities and dissimilarities.
The keys are based on the contrasting characters generally in a pair called couplet.
It represents the choice made between two opposite options.
This results in acceptance of only one and rejection of the other.
Each statement in the key is called a lead.
Separate taxonomic keys are required for each taxonomic category such as family, genus and species for identification purposes.
Keys are generally analytical in nature.