Among his major contributions to linguistic theory was the hypothesis that certain languages were non-configurational, lacking the phrase structure characteristic of such languages as English. Non-configurational languages, according to Hale, display a set of properties that cluster together, including free word order, unpronounced pronouns and the ability to disperse semantically related words across a sentence. Much of his research in the last two decades of the twentieth century was devoted to the development of syntactic models that could explain why these properties cluster. Hale's ideas initiated an important research program, still pursued by many contemporary linguists.