1993 Buick Roadmaster Owner's Manual - Page 157

Shape of Road Signs The shape of the sign will tell you something, too. ., I s An OCTAGONAL (eight-sided) sign-means STOP. It is always red with white letters. A TRIANGLE, pointed downward, indicates YIELD. It assigns the right-of-way to traffic on an certain approaches to intersection. .. A DIAMOND-shaped sign is a warning of something ahead-for example, a curve,steep hill, soft shoulder, or a narrow bridge.