clause is redeemable (often by the hiring firm). A small part of the students are admitted without having to pass an exam. Preparation for the concours takes place in preparatory classes which last two years (see grandes écoles). Other students called "normaliens étudiants" can be selected but they do not have their study paid. They are selected with the preparation of a research project (baccalaureate + 2–4 years). PhD students at ENS are either graduate students from the ENS doctoral school or from another doctoral school co-accredited by ENS. Since 2016 PhD students preparing their doctoral research at ENS are awarded a PhD from PSL University. ENS also welcomes selected foreign students (the "international selection"), participates in various graduate programs, and has extensive research laboratories. The foreign students selected often receive a scholarship which covers their expenses. The students selected via the concours remain at the school for a length of time ranging from four to six years. Normaliens from France and other European Union countries are considered civil servants in training. Many students devote at least one of those years to the agrégation, which allows them to teach in high schools or universities. Faculty recruitment is selective, with between zero and one ENS professorship open per year. Faculty recruitments usually happen upon previous incumbent retirements. In informal ENS jargon, ENS full professors are popularly called PdPs ("professeurs des professeurs)" because traditionally ENS was created to educate future professors. Divisions Founded to train high school teachers through the agrégation, ENS is now an institution training researchers, professors, high-level civil servants, as well as business and political leaders. It focuses on the association of training and research, with an emphasis on freedom of curriculum. The school's resources are equally divided between its "Letters" (social and human sciences and literature) and its "Sciences" (natural sciences and mathematics) sections. The school's fifteen departments and its 35 units of research (unités mixtes de recherches or UMR in French) work in close coordination with other public French research institutions such as the CNRS. The school has seven departments in its "Sciences" section: mathematics, physics, computer science, chemistry, biology, geoscience and cognitive science. It also has eight departments in its "Letters" section: philosophy, literature, history, classics, social science, economics (this section is the base of Paris School of Economics), geography, and art history and theory. In addition to these fifteen departments, a language laboratory for non-specialists