The present invention relates to a brush for applying a product to the eyelashes and/or eyebrows, that includes a core that extends along a longitudinal axis, and bristles held by the core, the core comprising a bristle-carrying portion with a proximal end, intended to be fixed to a stem, and a distal end, the bristles having free ends defining an envelope surface, the envelope surface having at least one cross section with a not entirely polygonal shape, the envelope surface defining at least one facet which extends longitudinally and is inclined with respect to the longitudinal axis of the core in the direction of the distal end, the facet having a width which increases in the direction towards the distal end of the bristle-carrying portion of the core, over at least a portion of the length of the facet.