The TIGER/Line shapefiles and related database files (.dbf) are an extract of selected geographic and cartographic information from the U.S. Census Bureau's Master Address File / Topologically Integrated Geographic Encoding and Referencing (MAF/TIGER) Database (MTDB). The MTDB represents a seamless national file with no overlaps or gaps between parts, however, each TIGER/Line shapefile is designed to stand alone as an independent data set, or they can be combined to cover the entire nation. A tribal block group is a cluster of census tabulation blocks within a single tribal census tract delineated by American Indian tribal participants or the Census Bureau for the purpose of presenting demographic data on their reservation and/or off-reservation trust land. The tribal block groups are defined independently of the standard county-based block group delineation. For federally recognized American Indian Tribes with reservations and/or off-reservation trust lands with a population less than 1,200, a single tribal block group is defined. Qualifying reservations and/or off-reservation trust lands with a population greater than 1,200 could define additional tribal block groups within their area without regard to the standard block group configuration. Tribal block groups do not necessarily contain tabulation blocks always beginning with the same number and could contain seemingly duplicate block numbers. Tabulation block numbers are still assigned by using standard block groups, not the tribal block groups. To better identify tribal block groups, the letter code range A through K (except I, which could be confused with a number 1) is used uniquely within each tribal census tract. The boundaries of tribal block groups and tribal census tracts are those delineated through the Tribal Statistical Areas Program (TSAP) for the 2010 Census.