Belmont County was first Washington County, Ohio as was most of the state.....Washington County came from the Virginia County of the same name which at one time contained all of Kentucky and parts of Tenn, and N C.
I do believe that Jefferson County evolved out of Washington County, the Belmont came from Jefferson.
The following link ( copy and paste it) will show you exactly how the counties were formed....just click on the years at the bottom of the map.
OOOps...I can't make it paste here...I'll send it to your personal mail.

Mike