A postulate concerning the structure of the transition state in a chemical reaction. It can be stated in the form that the structure of a transition state is like the structure of the chemical entity that is near to it in free energy. Thus in exothermic reactions the transition state is like a reactant, but in endothermic reactions it is like a product. This postulate was proposed by the American chemist George Hammond (1921–2005) in 1955 and is a useful guide for chemical reactions in organic chemistry.