A new method of analysing the linear complexity of 2nd-order nonlinear filterings of m-sequences that is based on the concept of regular coset is present. The procedure considers any value of the LFSR's length, L, (prime or composite number). Emphasis is on the geometric interpretation of the regular cosets which produce degeneracies in the linear complexity of the filtered sequence. Numerical expressions to compute the linear complexity of such sequences are given as well as practical rules to design 2nd-order nonlinear filtering which preserve the maximal linear complexity are stated.Este trabajo ha sido financiado por la Fundación Ramón Areces, por la Comisión Interministerial de Ciencia y Tecnología (CICYT) Proyecto TEL98-1020 y por la Comunidad Autónoma de Madrid Proyecto 07T/0044/1998.Peer reviewed