A thesis as a collection of articles or series of papers, also known as thesis by published works, or article thesis, is a doctoral dissertation that, as opposed to a coherent monograph, is a collection of research papers with an introductory section consisting of summary chapters. Other less used terms are "sandwich thesis" and "stapler thesis". It is composed of already-published journal articles, conference papers and book chapters, and occasional not yet published manuscripts. A thesis by publication is a form of compilation thesis (a term used in Nordic countries). Another form of compilation thesis is an essay thesis, comprising previously unpublished independent essays.