What's the difference between positive light and triangular light?

The light source of positive light completely comes from the side of the subject (the photographer faces the subject face to face), that is to say, the photographer makes a 90-degree angle with the light source. At this point, the dividing line between light and shade of the subject is in the middle, which is the so-called standard yin and yang face.

Triangular light is often emitted from the subject at an angle of 45 degrees (up, middle and down), which can form a triangular area near the nose of the face, and the proportion of the face is larger than that of the dark part, making the subject more stereoscopic. 45 degrees is not absolute, because the face may not be fixed on the front, or the head may turn to one side, so it needs to be adjusted when using light, so that the light and shadow of the face are in a triangular light state.