 Hoffmann doesn’t say much about the differing value of the horizontal and vertical explanations, but one important difference is that they give us different kinds of explanatory information. Vertical explanations demonstrate that chemical phenomena can be derived from quantum mechanics. They show that, given the (approximate) truth of quantum mechanics, the phenomenon observed had to have happened. Horizontal explanations are especially good for making contrastive explanations, which allows the explanation of trends. Consider again our example of the rate of an SN2 reaction. By appealing to the weakness of Br− as a base, the chemist invokes a chemical property, shared across other molecules. This allows her to explain methyl bromide’s reactivity as compared to methyl chloride, and also methyl fluoride, methyl iodide, etc. Insofar as chemists want to explain trends, they make contrastive explanations using chemical concepts.