Almada Negreiros was a Portuguese artist of the 20th century, one of the most relevant figures of Portuguese modernism. In the course his studies in visual art, he postulated the existence of a canon, a set of constants and geometric constructions underlying all art. Among these constructions were the n-th parts of the circle.
In this paper we consider the regular nonagon. It is known that there is no exact construction for this polygon using just straightedge and compass. Nevertheless, there are some very beautiful approximate constructions, which we describe in the first part of this paper. In the second part, we describe two constructions with a very small error. The last one is due to Almada Negreiros, one that he found in his description of the canon.