The Medical-Vocational Guidelines are used by Social Security to determine disability due to exertional impairments at step 5 of the sequential evaluation process. The guidelines, or the "grids," consider a claimant's exertional level (that's the medical part) and the claimant's age, education and work history (the vocational factors). Depending upon these medical-vocational factors, the SSA determines that a person is either disabled or not disabled.
In general, the grids are not where you want to be as a claimant, because the grids direct a finding of "not disabled" in most situations. In fact, every claimant loses under the grids until age 50 (or age 45 if unable to communicate in English).
However, the grids are a two-edged sword. Once you reach the age of 50 (and have no transferable skills or education that allows direct entry to skilled work), the grids direct a finding of "disabled" at the sedentary exertional level. The SSA presumes that the transition to unskilled sedentary work is too difficult for these claimants. At age 55, that same claimant grids "disabled" at the light exertional level.
Let's look at an example to see how the grids operate.