The first number assigned to each block identifies the block group of which it is a part. A block group may contain up to 97 blocks, but usually the number is substantially less. BGs generally contain between 250 and 550 housing units, with an average of 340. Block Blocks are the smallest census geographic area used by the Bureau of the Census for data tabulation and presentation. Block boundaries are usually streets and other prominent physical features.
Political divisions can also serve as block boundaries.
Blocks may be as small as a typical city block bounded by four streets or as large as several square miles in rural areas. The 1990 census is the first in which the entire country is block-numbered. In Census 2000, blocks will have a four-digit numeric code.