Block relationship files are state-based tabular files comparing the universe of 2010 Census blocks to the universe of 2020 Census blocks.

These block relationship files identify states, counties, tracts, and blocks by their FIPS (Federal Information Processing Series) or census supplied geographic codes. These files do not include the geographic names referenced by the FIPS codes. Block relationship files are provided in a pipe-delimited Unicode Transformation Format 8-bit block (UTF-8) encoded text file format. The record layout is provided below.

Block relationships may be one-to-one, many-to-one, one-to-many, or many-to-many.  The Block Part Flag indicates whether the record for an individual block within any pairing of blocks represents all or a portion of the block.  For instance, in a one-to-one relationship between a 2010 tabulation block and a 2020 tabulation block, the block part flag for each will be blank, indicating that the 2010 tabulation block in its entirety is coextensive with the 2020 tabulation block.  In a many-to-one relationship, multiple 2010 tabulation blocks may be contained within one 2020 tabulation block.  In this instance, the block part flag for each 2010 tabulation block would be blank; the block part flag for the 2020 tabulation block record in each pair would be “P” since the 2010 tabulation block relates to only a portion of the 2020 tabulation block.