It probably doesn't come as a surprise that maps today don't reflect accurately the areas where our ancestors lived long ago. Roads have changed. New towns have appeared (and some have disappeared). What you might not realize is that the counties themselves might have changed. Those changes can have a huge impact on your research. Most states didn't become states with their current number of counties. Ohio, which has 88 counties today, had only nine counties when it became a state in 1803. So how do you add more counties' You take the counties you have and chop them into smaller pieces. With each new county that is created comes a new place for records to be created. So what does this mean for our research' Let's say you have a family who shows up in Noble County, Ohio in the 1860 census. You look for them in 1850, but you can't find that family listed in Noble County. Turns out you're not going to find them in there in 1850, because Noble County wasn't formed until 1851. It might have been the county line that moved, giving your ancestors a different county of residence without ever leaving home. It's not just the census that is impacted by changes in county boundaries. Records stay where they were created. That means vital records, land records, court records ? any records created by the county government ? are going to stay in that county. When a new county was formed, they didn't go through the marriage records, for example, and say, "Ok, which of these weddings were performed in townships that moved over to the new county'" (Occasionally, you will find "new" counties that copied over land records from parent counties, but those are by far the exception.) The records stayed in the county where they were first created. The Genealogy of a County Learning when a county was formed and what county (or counties) was its "parent" will help you find the records. The Ancestry.com Wiki contains Red Book: American State, County, and Town Resources. Scroll down to the Table of Contents and click on the state you're interested in. On the new page, click on the link to that state's county resources. Here is the page for Ohio: You'll find a chart showing each county, when it was formed, the parent county/counties, and the dates that various records started in that county. Below is the top of the chart of the Ohio counties. Let's say you find your ancestors living in Auglaize County, Ohio in the 1850 census. The husband, wife and all of the children are listed as being born in Ohio. Based on the age of the children, it looks like the husband and wife were married in the early 1840s. Looking at the chart, we see that Auglaize County wasn't formed until 1848. If you don't find their marriage record there, the next places to look would be Allen and Mercer counties, because Auglaize was formed from them. When you're working on your genealogy and you're not finding your people in the area where you think they should be, take a look at the genealogy of the county. You might find that the county changed.