One of the procedures for making new words from the base of already existing words is "composition", which is much more common in English than in Spanish. This is due to the fact that English is more analytic while Spanish is more synthetic. While in English compounds are formed by the union of two or more free morphs, in Spanish one of the elements may differ from its free form, e.g. "pelirrojo", but the result in both languages is to create a word with new associations. This article seeks to analyse the different criteria used in determining compounds as well as to examine the relation between the different parts and their classification. It also seeks to establish that consequently, the classification procedure has to be applied differently in each language.