We have recently demonstrated that the electric near-field emerging from a triangular aperture at the tip of a metal-coated tetrahedron of glass exhibits a highly confined and intense spot when the light incident from within the glass body is polarized perpendicularly to one edge of the aperture [ Naber et al. Phys. Rev. Lett. 89 210801 (2002)]. Here we present additional experimental material and a theoretical confirmation of this near-field optical effect. Based on the model of a triangular aperture in a planar metal film, we studied the influence of aperture size and film material on the field distribution. Even though our theoretical model is rather simplified as compared to the experimental setup, we find our numerical calculations to be in very good agreement to the experimental results. The simulation confirms that it is mainly the reduced symmetry of the triangular aperture as compared to a circular one which leads to the enhanced confinement and strongly increased transmission of the electric field.