This article provides a formalization of the classification of intersection{x,y}* INTERSECTION{u, v }* of two monoids generated by two element codes. Namely, the intersection has one of the following forms {β, γ}* or (β0 + β(γ(1+δ+...+δt))* ε*).
Note that it can be infinitely generated. The result is due to [Karhumäki, 84]. Our proof uses the terminology of morphisms which allows us to formulate the result in a shorter and more transparent way.