Under normal circumstances, when both eyes look at an object, the image of the object will fall on the corresponding point of the retina of each eye (mainly in the macula). In children with strabismus, the eye position is skewed and the object does not fall on the corresponding point of both eyes, so the brain cannot integrate the object seen by both eyes, thus creating double vision. If you put a prism in front of the strabismic child’s eyes, when the degree is appropriate, through the refraction of the mirror, the objects seen by both eyes will fall on the corresponding point, and the double vision will disappear. According to this principle, trigonometry can be used to measure the degree of strabismus. Usually, trigonometry is performed in conjunction with “masking and de-masking” and “alternating masking. Trigonometry + masking de-masking: The child is asked to look at an object at 33 cm or 6 meters and the trigonometry is placed in front of the deviated eye. At this point, cover the eye looking at the object, observe whether the oblique eye behind the trigeminal prism moves, if there is movement, then keep increasing or decreasing the trigeminal prism degree, until the eye does not move. At this point, the trigonometry is the child’s strabismus angle. The trigonometry examination can determine the degree of strabismus and provide reference information for surgical design. However, due to the young age of the child with strabismus, it is sometimes difficult to accurately determine the degree of strabismus, so it is necessary to combine the measurements with other tests to determine the surgical plan.