We are often challenged with the question, “But what state or county was it called then?”  Go to:
See the date range in the upper left and enter a date within that range in the boxes in the upper right.  See the options on the right for the information to display and click “Refresh Map”. 
To change states, select a state in the window in the lower center screen
For optional state selection, change state name in the following:
This website provides an excellent guide to which county included a particular location on a particular date. 
So, if you are researching a family in county records or census records, you need to know the name of the county at that time for that location in order to know where to look for their records. 
I have heard of families who were listed in three different counties for three different censuses but never actually moved.  Only the county boundaries changed.
Also, if you don’t find the family where you think they should be, check a neighboring county.  It is possible that census takers or the families did not keep up with the county changes and continued voting, paying taxes or registering in the census in the wrong county.