The rule of thirds is a “rule of thumb” or guideline for composing photographs. It proposes that an image should be divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines. All the important elements of the photo should be placed along these lines or their intersections.
Two distinct, equal lights, should never appear in the same picture: One should be principal, and the rest sub-ordinate, both in dimension and degree: Unequal parts and gradations lead the attention easily from part to part, while parts of equal appearance hold it awkwardly suspended, as if unable to determine which of those parts is to be considered as the subordinate. “And to give the utmost force and solidity to your work, some part of the picture should be as light, and some as dark as possible: These two extremes are then to be harmonized and reconciled to each other.”
I remember learning about the 'rules of thirds' in photography school. But from time to time, you can get caught up in just trying to get the shot, without remembering that 'oh how should this photo be composed.' This is vital information, especially during a history changing event.