Basil Bernstein's theory about language codes and differential achievement by social class suggests that working-class pupils tend to only use the restricted code. That is the code of informal spoken English that often features colloquialisms and idiomatic turns of phrase as well as non-standard grammar and simplistic sentence structure. This is in contrast to middle-class pupils who can switch from the restricted code to the elaborate code which is the code used by teachers, textbooks and expected in assessed work.