As wider the scientific content as easier for teacher to know how to solve issues regarding his reality.
Chiss (1985) suggests teaching as a process of selection, reduction and simplification.
Transposition follows next didactic system
All the components work together depending on the other and becoming mutual.
The process of selection, reduction and simplification is used in text books, syllabus, manuals and pedagogic formative materials.
On the other hand, regarding Chevallard (1980) knowledge purpose is not to be taught but to be used in various situations. For this reason, it is necessary to teach the use of the language in real communicative contexts.