Those two-letter state abbreviations that the USPS—which I’m still tempted to style U.S.P.S. but won’t—likes to see on envelopes (MA, NY, CA, and the like) do not take periods. They also shouldn’t appear anywhere else but on envelopes and packages. In bibliographies and notes sections, and anywhere else you may need to abbreviate a state’s name, please stick to the old-fashioned and more attractive Mass., N.Y., Calif., and so on. Or just be a grown-up and write the whole thing out.
