From Ancestry.com Wiki
City directories are primarily useful for locating people in a particular place and time. They can tell you generally where an ancestor lived and give an exact location for census years. They are also useful for linkage with sources other than censuses.
There are usually several parts to a city directory. The section of most interest to the genealogist, of course, is the alphabetical listing of names, for it is there that you may find your ancestor. The other parts of a directory are equally important, however, as they will help you utilize the information contained in the alphabetical listings more efficiently. Street directories and ward boundary descriptions will be discussed in detail later in the chapter. There are also sections listing government offices, churches, civic and fraternal organizations, and businesses. These sections may be separated or combined.
Whenever you use a directory, however, it is important to refer to the page showing abbreviations used in the alphabetical section of the directory, usually following the name in each entry. Some abbreviations are quite common, such as h for home or r indicating residence. There may even be a subtle distinction between r for residents who are related to the homeowner and b for boarders who are not related.
Some city directories list adult children who lived with their parents but were working or going to school. Look for persons of the same surname residing at the same address. If analyzed and interpreted properly, these annual directories can tell you (by implication) which children belong to which household, when they married and started families of their own, and when they established themselves in business. In cases where a specific occupation is given, you can search records pertinent to that occupation.
There may be a page at the beginning of the alphabetical section that gives changes, additions, and deletions—usually due to removals, but sometimes due to death. Some city directories also give special facts—separate listings for African Americans or places of birth and death, for example, but such notations are usually for one year only and are not the norm.
Beginning in the late-nineteenth century, but more regularly in the twentieth century, city directories have often contained a “reverse” street directory, listing streets alphabetically, then the names of the people residing at each address. This is an important tool for identifying persons of different surnames residing at the same address. Another kind of “reverse” directory—by telephone number—has also become part of modern directories.
Once an ancestor has been found in a city directory, there are several ways the information can be used to gain access to, or link with, such sources as censuses, death and probate records, church records, naturalization records, and land records.