Let α > . By Cα we mean the terraced matrix de ned by c nk = n α if ≤ k ≤ n and if k > n. In this paper, we show that a necessary and su cient condition for the induced operator on l p , to be p-summing, is α > ; ≤ p < ∞. When the more general terraced matrix B, de ned by b nk = βn if ≤ k ≤ n and if k > n, is considered, the necessary and su cient condition turns out to be n n q p * | βn | q < ∞ in the region /p + /q ≤ .