George Hammond published a hypothesis in physical organic chemistry which describes the geometric structure of the transition state in an organic chemical reaction in his publication, Hammond's principle.
His 1955 publication asserted:
Therefore, the geometric structure of a state can be predicted by comparing its energy to the species neighboring it along the reaction coordinate.  For example, in an exothermic reaction the transition state is closer in energy to the reactants than to the products.  Therefore, the transition state will be more geometrically similar to the reactants than to the products.  In contrast, however, in an endothermic reaction the transition state is closer in energy to the products than to the reactants.  So, according to Hammond's postulate the structure of the transition state would resemble the products more than the reactants.  This type of comparison is especially useful because most transition states cannot be characterized experimentally.
Hammonds postulate also helps to explain and rationalize the Bell–Evans–Polanyi principle.  Namely, this principle describes the experimental observation that the rate of a reaction, and therefore its activation energy, is affected by the enthalpy change of that reaction.  Hammonds postulate explains this observation by describing how varying the enthalpy of a reaction would also change the structure of the transition state.  In turn, this change in geometric structure would alter the energy of the transition state, and therefore the activation energy and reaction rate as well.
The postulate has also been used to predict the shape of reaction coordinate diagrams.  For example, electrophilic aromatic substitutions involves a distinct intermediate and two less well defined states.  By measuring the effects of aromatic substituents and applying Hammond's postulate it was concluded that the rate-determining step involves formation of a transition state that should resemble the intermediate complex.
During the 1940s and 1950s, chemists had trouble explaining why even slight changes in the reactants caused significant differences in the rate and product distributions of a reaction.  In 1955 George S. Hammond, a young professor at Iowa State University, postulated that transition-state theory could be used to qualitatively explain the observed structure-reactivity relationships.  Notably, John E. Leffler of Florida State University proposed a similar idea in 1953. However, Hammonds version has received more attention since its qualitative nature was easier to understand and employ than Lefflers complex mathematical equations. Hammond's postulate is sometimes called the Hammond-Leffler postulate to give credit to both scientists.