What forms when a facial and lingual triangular ridge join?

The formation that occurs when a facial triangular ridge and a lingual triangular ridge join is known as a transverse ridge. This ridge typically appears on the occlusal surface of posterior teeth, specifically on molars. The transverse ridge aids in the function of the tooth by providing a structural element that helps facilitate the mastication of food.

Transverse ridges are important anatomically because they help in stabilizing the occlusion during chewing and contribute to the overall contour of the occlusal surface. In essence, they are key features that facilitate efficient food processing.

Other ridges, like the oblique ridge, form differently—typically from the joining of two triangular ridges on the same tooth but oriented at an angle. Marginal ridges are found at the mesial and distal edges of the occlusal surfaces of posterior teeth, while cusps are the pointed projections on the chewing surface. These definitions highlight why the concept of a transverse ridge specifically applies to the joining of facial and lingual triangular ridges, making it the correct answer in this case.