Used in multicomponent distillation, key components are the two main components that are required to be identified in a mixture to be separated. The two key components are the light key (LK), which is identified as the lightest component in the bottoms, and the heavy key (HK), which is the heaviest component in the distillate. They are known as key components since their identification is required to unlock the problem in solving the separation where the difficulty of separation, as measured in terms of the number of trays required for a given reflux ratio, is fixed by the concentration of the key (p. 207) components in the products. The relative volatilities of the components in a mixture are always determined with respect to the heavy key. Components that are lighter than the light key are termed LLK while those that are heavier than the heavy key are termed HHK.