Sometimes even the best hunters do not always find what they are seeking. Tracking your forebearers may require the use of substitute records. Often, an alternative objective may be acceptable.
For example, the ultimate goal is to find an ancestor's birthplace in a foreign country. Someone tells you that a birth record would give the birth place. Obviously, you would go hunting for a birth record. Then you discover that you need the birth place to find the birth record. This seems like a vicious circle.
But then you learn that a marriage record might give the place of birth. In this instance, a marriage record is a substitute record for the birth. But you don't know where the couple was married, or even how long they were married.
Then you learn that a 1900 census provides the number of years a couple was married. Your ancestor was an adult in that time period and might be listed. You find that this record not only provides the year of marriage but the country of birth. Part of your goal to find the birth place is accomplished.
Then you learn that a foreign newspaper obituary has a greater likelihood of providing a place of birth in the old country than an American newspaper, so you go hunting for foreign newspapers in the language and within the geographical location of your ancestor in America.
Therefore, the hunt continues from one record to another as you pick up pieces of the information to solve your genealogy research problem. There are other substitutes of which you should also be made aware.