A closed thesis is a statement of the main idea of the argument that also previews the major points the writer intends to make. It is “closed” because it limits the number of points the writer will make. For instance, here is a closed thesis on the appeal of the Harry Potter book series:
The three-dimensional characters, exciting plot, and complex themes of the Harry Potter series make them not only legendary children’s books but enduring literary classics.
This thesis asserts that the series constitutes a “literary classic” and specifies three reasons—characters, plot, and theme—each of which would be discussed in the argument. A closed thesis often includes (or implies) the word because. This one might have been written as follows:
The Harry Potter series have become legendary children’s books and enduring literary classics because of their three-dimensional characters, exciting plots, and complex themes.
Indeed, that statement might be a good working thesis.
A closed thesis is a reliable way to focus a short essay, particularly one written under time constraints. Explicitly stating the points you’ll make can help you organize your thoughts when you are working against the clock, and it can be a way to address specific points that are required by the prompt or argument.