I'm not sure of the meaning of "twice twice" and "twice on itself" respectively. My guess would be for exercice 3 that I should do something like : typedef twice::type T1; // T1 would be T ** // The second time, I replace T (in to_be_fiiled_1) by // T1 (in to_be_filled_2) , thus T2 is T1** which is also T**** typedef twice::type T2; while for exercice 4, we want something that would look like : typdef twiceto_be_filled_3b>::type T3; Does I understand the questions correctly? For twice, I use the definition from section 3.3.2. Philippe