The Maddox cross is a tangent scale for squint angle measurement and retinal correspondence testing. In association with rod can be used to subjectively detect and measure a latent (heterophoria), manifest, horizontal or vertical strabismus for near and distance.
The tangent scale is cross-shaped, sometimes its vertical arms are shorter. In the middle of the cross at the zero point, there is a small white light.
On the arms of the cross, there are scales marked with numbers. The horizontal arm of the cross is usually about 1 m long, allowing to examine eye deviations up to 10o with examination distance of 5m, and up to 5o with distance of 2 metres. Individual distances of the scales correspond to radians. The larger one is used for distance of 5 m, and the smaller one for distance of 1 m.