 As an example Granger gives three ways of introducing the complex numbers; all three ways account for the structural properties which characterize the algebraic structure in question. The first way introduces the complex numbers by trigonometric representation using angles and directions. The second introduces them as operators applied to vectors. In the first case, one defines a complex number as a pair of real numbers and the additive properties are then immediate. By contrast, in the second case, it is the multiplicative properties that are immediately seized. But, and this is the third way, one can also introduce complex numbers by regular square matrices. This leads to seeing the complex numbers as a system of polynomials in x modulo x2+1.