The triangular face shape is less common than the above mentioned face shapes. And often not as close to the geometrical shape as the aforementioned. The characteristics are that the face has a strong jawline like the rectangular shape, but with the triangular shape the forehead is narrower that the jawline.
Sometimes it is also referenced as a drop shaped face.
So to achieve harmony we wish to add some strength to the upper part of the face. Socalled browline glasses are perfect as they have a significant upper rim and a subtle rim for the rest. This effect is also achieved with gradient materials that are darker at the upper frame and get gradually lighter downwards. Rimless frames could also be a great option for triangular faces.