Auguste Herbin (1882-1960) was one of the great pioneers of geometric constructivist art in France. His main work is the unique "Alphabet Plastique", which comprises a code embodying the letters of our alphabet as well as colors, forms and musical notes. Together we will study several of his originals and get to understand the "Alphabet Plastique". We will see that, in spite of his strict adherence to his coding principles, Herbin also allowed himself a great deal of freedom in the conception of his pictures. Using his "Alphabet Plastique", any child is able to encode its name and turn it into a work of art. Together we will, at the end, compose a large Arithmeum picture.