Geographers often seek to categorize places using a scale of development, frequently dividing nations into the "developed" and "developing," "first world" and "third world," or "core" and "periphery." All of these labels are based on judging a country's development, but this raises the question: what exactly does it mean to be "developed," and why have some countries developed while others have not? Since the beginning of the twentieth century, geographers and those involved with the vast field of Development Studies have sought to answer this question, and in the process, have come up with many different models to explain this phenomenon.