Trigonometry with alternating masking can be used completely for children over 2 years of age, and for children between 1 and 2 years of age some can be used. To check the degree of strabismus at a distance, the long vertical plane of the trigeminal prism must be parallel to the eyelid, not to the cornea of the strabismic eye; to check the strabismic angle at a close distance, the prism should be as close to the cornea as possible (so as not to harm the cornea), and the prism should be on the nasal side.