The standard STOP sign used in the United States has eight sides.
The octagonal shape is deliberately distinctive, allowing drivers to recognize the sign from behind or when snow, darkness, or dirt obscures its lettering. The United States adopted the octagon for stop signs in 1923, and the shape later became widely associated with stopping internationally.
The red background and white lettering are also important parts of the modern sign. Earlier American stop signs used yellow backgrounds, but red became standard after improvements in durable reflective materials and greater international color standardization.
An eight-sided sign is not the same as a yield sign, which is an inverted triangle. At a stop sign, drivers must stop at the marked limit line, before a crosswalk, or before entering the intersection, depending on the road layout and applicable law.