A glasses prescription is the little piece of paper that has your spectacle correction written on it. There are many numbers on a glasses prescription and they can be a little daunting to decode if you’ve never done this before. The first number is your basic refractive error: if you are farsighted this will be a positive number and if nearsighted this will be a negative number. The second number is how much astigmatism you have (if you have any). The last number is a degree measurement used to align the astigmatism glasses in their frames properly. Finally, the “add” is the power of any bifocal correction and can range anywhere from +1.00 to +3.50 depending upon your age and how close to your face you like to read books. OD stands for right eye and OS is the left eye.