The Census 2000 census tract relationship files are in fixed length ASCII format. The two census tract relationship files consist of one record per each 1990 census tract/2000 census tract spatial set. A spatial census tract set is defined as the area that is uniquely shared between a 1990 census tract and a 2000 census tract. The two files of significantly changed census tracts consist only of a list of census tracts that exhibited a change of 2.5-percent or greater. The current release includes 56 sets of files, one set for each of the 50 states plus separate files for the District of Columbia, Puerto Rico, American Samoa, Guam, the Northern Mariana Islands, and the U.S. Virgin Islands.

The file naming convention is in parenthesis below after the file name. The "xx" represents the 2-character Post Office state abbreviation and "NN" represents the 2-digit state FIPS code.

Population-based Census Tract Relationship File Layout (xxNNpop.txt)

Field	Field Length	Field Description
1-2	2	1990 state FIPS code
3-5	3	1990 county FIPS code
6-9	4	1990 census tract base
10-11	2	1990 census tract suffix
12	1	1990 census tract part flag
13-21	9	Census 2000 population for 1990 census tract
22-25	4	Percentage of 1990 census tract population^*
26-27	2	2000 state FIPS code
28-30	3	2000 county FIPS code
31-34	4	2000 census tract base
35-36	2	2000 census tract suffix
37	1	2000 census tract part flag
38-46	9	Census 2000 Population of 2000 census tract
47-50	4	Percentage of 2000 census tract population^*
51-59	9	2000 population of the area covered by the record
60-73	14	Land area of the record (1000 sq.meters)
74-75	2	2000 state name abbreviation
76-135	60	2000 county name

^*These figures are shown to one decimal place (000.1); the decimal place is implied.

Street-side Mileage-based Census Tract Relationship File Layout (xxNNssm.txt)

Field	Field Length	Field Description
1-2	2	1990 state FIPS code
3-5	3	1990 county FIPS code
6-9	4	1990 census tract base
10-11	2	1990 census tract suffix
12	1	1990 census tract part flag
13-18	6	Street-side mileage of 1990 census tract
19-22	4	Percentage of 1990 census tract street-side mileage^*
23-24	2	2000 state FIPS code
25-27	3	2000 county FIPS code
28-31	4	2000 census tract base
32-33	2	2000 census tract suffix
34	1	2000 census tract part flag
35-40	6	Street-side mileage of 2000 census tract
41-44	4	Percentage of 2000 census tract street-side mileage^*
45-50	6	Street-side mileage of the area covered by the record
51-64	14	Land area of the record (1000 sq.meters)
65-66	2	2000 state name abbreviation
67-126	60	2000 county name