You have to enter something in that box. You can enter the county, or locality that the tax is for. If you don't know, ask your employer.
This is the name of the locality responsible for assessing your local income tax. It could be a city, town, school district, or something else. If you have an amount in box 18 (Local wages & tips) or box 19 (Local income tax), then you need to enter the locality name in box 20.
Depending on where you live, you may even have more than one locality name. If you do, just add a new row to enter info for another locality.
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