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Q. Why does a comma follow Washington, DC, in running text?
A. That second comma helps to set “DC” off from the surrounding text; together, the commas work like parentheses. (Like parentheses, such commas always come in pairs.) Consider that without the second comma, a misreading is possible. For example, “Washington, DC is a great city” might mean that you are telling someone named Washington that DC is a great city. Another option is to omit both commas: “Washington DC is a great city.” That’s US Postal Service style for mailing labels, but it’s not Chicago style for running text (though maybe one day it will be).