The inverted pyramid is a metaphor used to illustrate how information should be arranged or presented within a text, in particular within a news story.
The “pyramid” can also be drawn as a triangle. The triangle’s broad base at the top of the figure represents the most substantial, interesting, and important information the writer means to convey. The triangle’s orientation is meant to illustrate that this kind of material should head the article, while the tapered lower portion illustrates that other material should follow in order of diminishing importance.
The format is valued because readers can leave the story at any point and understand it, even if they don’t have all the details. It also allows less important information to be more easily removed by editors so the article can fit a fixed size.