Vertical direction refers to the up and down direction of smiling expression, that is, the angle at which the corners of the mouth rise. Vertical factors have an important influence on the aesthetic evaluation of smiles, which are embodied in the following aspects:

1. vertical angle: the vertical angle of smile has a significant impact on aesthetic evaluation. Research shows that when the vertical angle is between 15 and 25 degrees, it is considered as the most beautiful smile angle.

2. Up and down symmetry: The up and down symmetry of smiling expression is also one of the factors that affect aesthetics. Studies have found that symmetrical smiles are more popular than asymmetrical smiles.

3. Lip shape: Lip shape will also affect the aesthetic evaluation of smile. Research shows that the thickness, length and width of lips when smiling will affect the aesthetic evaluation of smiling.

4. Gingival exposure: Gingival exposure when smiling will also affect aesthetic evaluation. It is considered as the most beautiful smile when the gums are exposed between 1 and 2 mm

Smile is the most basic, universal and popular expression in human expression, which has positive social and emotional expression functions. Smile aesthetics refers to people's aesthetic evaluation and cognition of smile expression, which is influenced by many factors, including vertical factors.

In a word, the longitudinal factors of smile aesthetics have an important influence on aesthetic evaluation. In the design and aesthetic evaluation of smiling expression, the influence of vertical factors should be considered in order to obtain better aesthetic effect.