An assembly of a stiffener and a composite panel, the stiffener comprising: a plank, the plank having an elongate configuration with a longitudinal length dimension, a lateral width dimension and a thickness dimension; a bottom sub-plank in the plank, the bottom sub-plank being constructed of layers of composite material; a first layer of adhesive on the bottom sub-plank; and, an intermediate sub-plank in the plank, the intermediate sub-plank being constructed of layers of composite material, the intermediate sub-plank engaging against the first layer of adhesive with the first layer of adhesive securing the intermediate sub-plank to the bottom sub-plank; the composite panel having an elongate configuration with a longitudinal length dimension, a lateral width dimension and a thickness dimension, the composite panel being constructed of layers of composite material; the plank being secured to the composite panel; and, layers of tapes of composite material being in the composite panel, the tapes of composite material being interleaved in the layers of composite material of the composite panel, the tapes of composite material extending along the longitudinal length dimension of the composite panel and along the longitudinal length dimension of the plank, the tapes of composite material being positioned in the thickness dimension of the composite panel only beneath the plank on the composite panel. the stiffener comprising: a plank, the plank having an elongate configuration with a longitudinal length dimension, a lateral width dimension and a thickness dimension; a bottom sub-plank in the plank, the bottom sub-plank being constructed of layers of composite material; a first layer of adhesive on the bottom sub-plank; and, an intermediate sub-plank in the plank, the intermediate sub-plank being constructed of layers of composite material, the intermediate sub-plank engaging against the first layer of adhesive with the first layer of adhesive securing the intermediate sub-plank to the bottom sub-plank; a plank, the plank having an elongate configuration with a longitudinal length dimension, a lateral width dimension and a thickness dimension; a bottom sub-plank in the plank, the bottom sub-plank being constructed of layers of composite material; a first layer of adhesive on the bottom sub-plank; and, an intermediate sub-plank in the plank, the intermediate sub-plank being constructed of layers of composite material, the intermediate sub-plank engaging against the first layer of adhesive with the first layer of adhesive securing the intermediate sub-plank to the bottom sub-plank; the composite panel having an elongate configuration with a longitudinal length dimension, a lateral width dimension and a thickness dimension, the composite panel being constructed of layers of composite material; the plank being secured to the composite panel; and, layers of tapes of composite material being in the composite panel, the tapes of composite material being interleaved in the layers of composite material of the composite panel, the tapes of composite material extending along the longitudinal length dimension of the composite panel and along the longitudinal length dimension of the plank, the tapes of composite material being positioned in the thickness dimension of the composite panel only beneath the plank on the composite panel.