The changing of political boundary lines, especially those involving Counties in the United States, is something that needs to be considered whenever a researcher begins to look for records that pertain to some specific ancestor. For example, in 1900 Nassau County, New York was created out of Queens County. Therefore, if your ancestor lived in the Town of Hempstead at the turn of the 20th century (from the 19th), he/she would have began life living in Queens County, while ending it in Nassau County, all without ever having moved his or herself. Searching for records of the ancestor's birth or marriage would require looking at the records held by courts in Queens County. Then, when it comes time to look for death or probate records, the search would move to Nassau County. A deed for property purchased in 1899 should be found in Queens County, while a sale for the same property in 1901, might be recorded in Nassau County. Not knowing that Nassau County was created out of Queens County in 1900 could hinder your search strategy.
Political jurisdictions and boundaries have changed continuously throughout the history of the United States. This was especially true, and frequent, during the first half of the 1800s. Knowing where to look for records, depending on the time period in question, is the first step in a successful record search. Thankfully, there is a website, Atlas of Historical Boundaries (Newberry Library), that is easy, even fun to use, and indispensable for tracking down county formation and changes. Start by simply selecting a state from the map on the home page. The one I use most often is New York. At this individual state page you will find a number of different tools to use, some of which can be downloaded and used offline. My favorite is the Interactive Map. With New York you are taken to a map of the State with the current counties outlined in white and an overlay (outlined in black) of the counties as they existed on July 4, 1776. The tools to the right allow you to manipulate the map in any many different ways. The "Select Map Date" allows you to view the county boundaries of the state at any particular point in time. For exact dates regarding changes and county formations, the County Index page is indispensable. Just select a county and you'll find its entire history of changes that effected its boundaries. The Consolidated Chronology provides a time line for boundary changes statewide beginning, in the case of New York, in 1606 when charters were granted by King James I.
The Atlas of Historical Boundaries website is easy to use and is an extremely useful tool that all genealogy researchers should be aware of.