When census tracts are established, they are designed to be relatively homogenous areas with respect to population characteristics, economic status, and living conditions. Tract boundaries are established with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. When sufficient population increases occur, tracts are subdivided. It is easy to recombine the new tracts for comparison with older tracts. Other changes have included combining two or more tracts and minor realignments of tract boundaries to reflect new or better recognized physical features.
Census tracts observe county lines and cover all of the territory within each tracted county.
Within a county, tract boundaries may be split by other geographic boundaries. In the case of independent cities, like the City of St. Louis, which is not in a county, the independent city is treated like a county and appears at the end of all of the county tables. They are the second smallest type of census area. These are the smallest type of census area. They average about 70 people and most commonly are small rectangular areas bounded by four streets.