The Great Pyramid of Giza has eight sides, rather than four. All of the other pyramids have just four sides.
One very unusual feature of the Great Pyramid is a concavity of the core that makes the monument an eight-sided figure, rather than four-sided like every other Egyptian pyramid. That is to say, that its four sides are hollowed in or indented along their central lines, from base to peak. This concavity divides each of the apparent four sides in half, creating a very special and unusual eight-sided pyramid.
The hollowing-in can be noticed only from the air, and only at certain times of the day. This explains why virtually every available photograph of the Great Pyramid does not show the hollowing-in phenomenon, and why the concavity was never discovered until the age of aviation.
Apparently, the eight-sides were discovered entirely by accident in 1940 when a British Air Force Pilot, P. Groves, flew over the pyramid and realized the concavity, taking a picture that is now famous among those who are into this sort of thing.