The present invention relates to a stringer, particularly to a stiffening and structural longitudinal element for skin of aircraft airfoil surfaces, characterized in that it incorporates two separate and independent reinforcement areas, one in the foot and the other at the end of the web (head) of the stringer, where the reinforcement areas are obtained by a widening of the section of the stringer, the widening being formed by a localized stack of reinforcing plies of composite material which are embedded within the stringer. The invention also relates to the method of manufacture and is particularly applicable to stringers having an L-shaped or T-shaped section.