The two primary types of cover tests are the alternating cover test and the unilateral cover test (cover-uncover test).
The cover test is used to determine both the type of ocular deviation and measure the amount of deviation. The two primary types of ocular deviations are the tropia, also known as *Strabismus, and the phoria. A tropia ia a constant misalignment of the visual axes of the two eyes, i.e. they can't point the same direction. A phoria is a latent deviation that only appears when fixation is broken and the two eyes are no longer looking at the same object.
The unilateral cover test is used to determine whether the deviation is a phoria or tropia, and the alternating cover test then used to measure the amount of deviation, usually with the aid of loose prisms.