The origin of the sonnet, invented about 1230/40 within the circle of the learned officials at the Sicilian court of the Emperor Frederick II, is one of the most discussed problems of medieval Italian literature. The essence of the question consists in defining the intrinsic aesthetic quality produced by the typical meter of 14 rhymed hendecasyllables and its original structure determined by the ratio 8 : 6 = 4 : 3. The following essay presents a geometrically based theory about the specific measures defining the metrical form as well as some graphical variants of the lyrics used in the first Italian canzonieri. The major formulas relevant in these regards are 14 • 11 = 154, 7 • 22 = 154 and (4 • 22) + (2 • 33) = 154. All these numeric terms can be related to some central mathematical items described in numerous medieval treatises, especially in the works of Fibonacci (about 1180–1241), a friend of the first sonnet writers. By means of the scientific texts it can be shown that the metrical and graphical measures of the classical sonnet are identical with the components of fractions constituting approximations to the values of p (the key of the quadratura circuli ) and 1/f (the ratio called the golden section). As p and f are irrational numbers, i.e., fractions that cannot be expressed as terminating or repeating decimals, the formal structure of the primo sonetto is revealed as an invention inspired by the idea of creating an iconic diagram representing the typical concept of a sonnet: the poet’s adoration of an unattainable lady. In a short excursus it will be pointed out that the number 154 as the constant element in the basic laws of the Italian sonnet is identical with the sum of the lyrics included in Shakespeare’s quarto-edition of 1609. The amazing fact of this numerical concordance leads us to the hypothesis that, in spite of the well-known formal differences in his poetic meter, the most famous English sonnet writer may have intended to pay homage to his Sicilian predecessors by means of the key number 154.