I have seen a lot of questions on message boards about whether a place is in Poland or if a particular person is actually from Poland. This is a common question among Polish researchers and the answers have a lot to do with the time period being researched.
For me, this question started with my great, great grandfather, who arrived in the United States around 1888 or 1889. I have located him in the 1910, 1920, and 1930 Federal Census records. In 1910, his place of birth is listed as "Ger/Polish." The, in 1920, his place of birth is "Posen" and his native language is "Polish," although the Ancestry.com index shows his place of birth as "Germany." Finally, the 1930 census clearly shows his place of birth as "Poland." So how could the same person be born in multiple counties...or why does his birth place change depending on when he is asked?
The answer lies in Poland's history. Over the course of time and even in our lifetimes, the borders and names of what we know today as Poland have changed. In 1910, Poland was not recognized--rather, Polish lands had been divided among Prussia, Russia, and Austria. A birth place of "Ger/Polish" would indicate Prussia, or the German Empire. In 1920, these lands were once again known as Poland, which explains the response of Posen/Polish. Although Posen (Poznan) is a clue, it is a large city and does not necessarily indicate where my ancestor was born or lived before moving to the United States. For example, he could have been from a much smaller town, but indicated Posen, as it was the largest city close to where he actually lived. It is important to note, however, that Posen was part of the German Empire in 1910. Finally, 1930 was prior to another division of the country by Nazi Germany, so it was still known as Poland.
It is helpful to consult a historical map in locating the origins of your Polish ancestors. For the time period I am researching, I referenced this Historical Map of Europe 1871-1914, at EmersonKent.com, which shows city names, as well as color-coding for the controlling territory. For example, Posen is part of the German Empire (in blue), situated between Berlin and Warsaw, which was part of Russia at the time.
Understanding land divisions of Poland during your ancestor's lifetime and consulting historical maps is extremely beneficial in determining where they lived. Be careful not to overlook clues in census, immigration, and other records created by your ancestor. Records are likely to refer to the location as it was known to the rest of the world at that point in time.