From the vegetable structure of the coconut to the diagram of human settlements by Le Corbusier, one can frequently find the shape of the equilateral triangle in many different occurences, both in a natural environment and in artificial works. Along with the circle and the square, the equilateral triangle is one of the three basic forms, and is suitable to be combined in modular frameworks to generate a structured field in which endless other combinatorial forms may be constructed. From classical Arab and Japanese decorations to the contemporary architecture of Buckminster Fuller and Wright, the familiarity with the equilateral triangle, in all its formal and structural resources, generates curious and fascinating experimentations. After the books of the same collection dedicated to the circle and the square, a new reprint by Bruno Munari about the many uses of this evocative shape throughout the centuries. These studies were originally published in 1976 in the series Quaderni di design, curated by Munari himself for Zanichelli.