Washington County, D.C.

When working with deeds for Washington D.C., Prince Georges and Montgomery County. You may come across property located in Washington County, D.C.. This means the land was located outside of the early Washington D.C. city limits and Georgetown. The designation was in use from 1801 to 1871 when it was abolished.

https://en.wikipedia.org/wiki/Washington_County,_D.C.