 In contrast with vertical mode, the horizontal mode of explanation attempts to explain chemical phenomena with chemical concepts. For example, all first year organic chemistry students learn about the relative reaction rates of different substrates undergoing the SN2 reaction. An organic chemist might ask “Why does methyl bromide undergo the SN2 reaction faster than methyl chloride?” One answer is that “the leaving group Br− is a weaker base than Cl−, and all things being equal, weaker bases are better leaving groups.” This explains a chemical reaction by appealing to a chemical property, in this case, the weakness of bases.