This is a lens ground into a pair of glasses that is designed to bend light and help alleviate double vision. With double vision (also known as diplopia), people see two of the same object. This is usually because of an alignment problem between the eyes such as when someone is cross-eyed. To help alleviate this doubling, prism correction can be ground into a pair of spectacles to bend the light in such a way that the doubling goes back to “normal.” Prism glasses are effective for many people, but they have some limitations. Prism adds significant thickness and cost to the glasses. They are also difficult to get “just right” and may require many visits and possible remakes (frustrating for both the patient and the optical shop). Also, some people have alignment problems that vary throughout the day … they may seem fine in the morning, but as the day progresses their double vision worsens. Prism glasses don’t work well in this situation as the correction needed is constantly changing. Finally, some people have double vision that is more pronounced at distance, or near, or when looking in a particular direction. Once again, prism glasses can only correct a ‘single’ alignment problem and so several pairs of glasses may be required (such as separate distance and reading glasses).