[1] Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary.

[3] Diplopia can be one of the first signs of a systemic disease, particularly to a muscular or neurological process,[4] and it may disrupt a person's balance, movement, or reading abilities.

Doctors may use blood tests, physical examinations,[13] computed tomography (CT), or magnetic resonance imaging (MRI) to find the underlying cause.

Inadequate diagnosis and treatment at improper times and tethering or fibrosis of muscles may lead to persistent diplopia.

[16] Binocular diplopia is double vision arising as a result of strabismus[17] (in layman's terms "cross-eyed"), the misalignment of the two eyes relative to each other, either esotropia (inward) or exotropia (outward).

[18] The brain calculates the visual direction of an object based upon the position of its image relative to the fovea.

Certain people with diplopia who cannot achieve fusion and yet do not suppress may display a certain type of spasm-like irregular movement of the eyes in the vicinity of the fixation point (see: Horror fusionis).

[22] If diplopia appears with other symptoms such as fatigue and acute or chronic pain, the patient should see an ophthalmologist immediately.

[25] Monocular diplopia may be induced in many individuals, even those with normal eyesight, with simple defocusing experiments involving fine, high-contrast lines.

If diplopia turns out to be intractable, it can be managed as last resort by obscuring part of the patient's field of view.

This approach is outlined in the article on diplopia occurring in association with a condition called horror fusionis.