The simple cover and cover-uncover tests may be used to demonstrate manifest and latent strabismus (heterotropia and heterophoria) respectively.
The cover test demonstrates tropias: the uncovered eye is forced to adopt fixation; any movement therefore represents a manifest strabismus (heterotropia).
The cover-uncover test demonstrates phorias: any movement of the covered eye to reestablish fixation as it is uncovered represents a latent strabismus (heterophoria). The alternate cover or cross cover test, in which the hand or occluder moves back and forth between the eyes, repeatedly breaking and reestablishing fixation, is more dissociating, preventing binocular viewing, and therefore helpful in demonstrating whether or not there is strabismus. It should be performed in the nine cardinal positions of gaze to determine the direction that elicits maximal deviation. However, it does not distinguish between tropias and phorias, for which the cover and cover-uncover tests are required.