It occurred to me that several useful analyses could be provided someone trying to track down a family. In the short time I was in the library I thumbed through successive years for a particular person and found his changes in profession and where he moved to over about a ten-year period. I made several discoveries by studying particular groupings of surnames in the directory. Relationships, kinships, and marriages which had not been previously evident, were found by comparing each year to the next year. Names sometime change from year to year, giving a good insight into the variations of spelling and the use of nicknames. A researcher can also follow up on what you find in the directories by researching your relative's employer. Such research can sometimes lead to other sources like funeral homes and local newspaper articles.
You can identify neighbors by using the cross index of streets. As in the countryside, neighbors may be related or are family friends. Do not forget to check for city businesses, schools and organizations as well as neighborhood churches. If they lived in a particular neighborhood, they may have records in that local area. A family disappearing from the listing may mean they moved inside or outside the city, or that they could of died. This may lead to more sources of information.
City directories are often the first collections of most historical and genealogical societies. The Library of Congress has a large collection of city directories on a variety of formats which include microfiche, paper, and electronic. Over 1,000 cities from 1861 to 1960 can also be found entitled: "City Directories of the United States," published by Primary Source Microfilm. There is also a separate microfiche collection of pre-1861 U.S. directories, available in the Microfilm reading room at the Library of Congress.
Online at San Francisco. I found a listing of ten major cities with the location and address of where the directories can be found. It also lists the Family History Center Library of Salt Lake City as having directories on microfilm as early as 1850.
Directories are a wonderful and often overlooked source which can open the door about your family's history. The only limit to what can be found is the number of years in the past in which the directory was published.