N.H. Rev. Stat. Ann. § 22:7

The county of Hillsborough is bounded thus: Beginning at the bound between the towns of Pelham and Salem on the state line; thence westerly by the state line to the southeast corner of Rindge; thence by the easterly lines of Rindge, Jaffrey, Dublin, Nelson, Stoddard and Washington to the northwest corner of Hillsborough; thence by the southerly line of the county of Merrimack to the westerly line of the county of Rockingham; thence by the westerly line of the county of Rockingham to the bound first mentioned.