I do quite a bit of sourcing 1600s and 1700s profiles in Connecticut. Very frequently, those unsourced profiles list Middlesex County as place of birth, marriage or death for the towns of Middletown, Chatham, Haddam, East Haddam, Saybrook, Killingworth, and Durham. That is true only of the first five towns in 1785 and later, and of Durham in 1799 and later. I am often, therefore, correcting the location to the actual county for events occurring prior to those dates. I thought this might be of interest to those who were not aware of it.
"Middlesex county was incorporated by an act of the Legislature passed at the May session, 1785, and at that time consisted of six towns. Of these, Middletown, Chatham, Haddam, and East Haddam were taken from the county of Hartford, and Saybrook and Killingworth from New London county. Durham was annexed to the county in May 1799, from the county of New Haven." from "The History of Middlesex County 1635-1885" (J. H. Beers & Co., 36 Vesey Street, New York; 1884) transcribed by Janece Streig. Dunham, Wilcox