Location Names and Boundaries
Many cities, counties, and states didn't always have the names that we associate them with today. For example, New Amsterdam is now called New York City. In addition, the boundaries of a city, town, county, or country may have changed over time. Your ancestors may have moved to another county without ever packing their bags!
It is boundary changes that can make locating your ancestors' records difficult, because the records almost always stayed in their original location -- the county seat of the parent county. This means that if a boundary change caused your ancestors to make a "move" that you don't know about, you may be looking in the wrong place for their records. You may also find it difficult to locate someone born in a territory before it became a state -- a person's records could be in any one of the states that that territory became.
In short, it may be helpful to learn the history of boundary changes in the area where you suspect that your ancestors lived. Three good books for learning about boundary changes are George B. Everton's The Handy Book for Genealogists and William Thorndale and William Dollarhide's Map Guide to the U.S. Federal Censuses, 1790 to 1920 .