Let C 1 and C 2 be two (classical) [ n , k 1 ] , [ n , k 2 ] codes such, that C 2 ⊂ ⊂ C 1 and C 1 , C 2 ⊥ ⊥ both have minimal distance ≥ ≥ 2 t + 1 , where C 2 ⊥ ⊥ is the code dual to C 2 . Then define CSS ( C 1 , C 2 ) , the CSS code of C 1 over C 2 as an [ n , k 1 − − k 2 , d ] code, with d ≥ ≥ 2 t + 1 as follows: