Optics for the mathaphobic--prism mania.
When combining like values, add and take the sign of those numbers. When combining values that are not alike, subtract and take the sign of the higher number. Draw the numbers on an optical cross or number line to see what the numbers mean. Don't panic. Prism can be a blessing to the patient with diplopia.
Friedman
K L
KL
D005139:Eyeglasses; D006801:Humans; D008433:Mathematics; D055095:Optics and Photonics; D009907:Optometry / Q000379:methods*; D055656:Prescriptions