The Dewey Decimal system used for numbering books by U.S. state was designed to group the states by geographical regions. The numbers for the regions are:

	Northeastern States	974
	Southeastern States	975
	South Central States	976
	North Central States	977
	Western States	978
	Pacific States	979

Each U.S. state is assigned its own Dewey Decimal number within its regional grouping number. Within each U.S. state, each county is assigned its own unique Dewey Decimal number. The Dewey Decimal numbers for the U.S. states are:

Alabama	976.1	Louisiana	976.3	Ohio	977.1
Alaska	979.8	Maine	974.1	Oklahoma	976.6
Arizona	979.1	Maryland	975.2	Oregon	979.5
Arkansas	976.7	Massachusetts	974.4	Pennsylvania	974.8
California	979.4	Michigan	977.4	Rhode Island	974.5
Colorado	978.8	Minnesota	977.6	South Carolina	975.7
Connecticut	974.6	Mississippi	976.2	South Dakota	978.3
Delaware	975.1	Missouri	977.8	Tennessee	976.8
Florida	975.9	Montana	978.6	Texas	976.4
Georgia	975.8	Nebraska	978.2	Utah	979.2
Hawaii	996.9	Nevada	979.3	Vermont	974.3
Idaho	979.6	New Hampshire	974.2	Virginia	975.5
Illinois	977.3	New Jersey	974.9	Washington	979.7
Indiana	977.2	New Mexico	978.9	West Virginia	975.4
Iowa	977.7	New York	974.7	Wisconsin	977.5
Kansas	978.1	North Carolina	975.6	Wyoming	978.7
Kentucky	976.9	North Dakota	978.4	D. C.	975.3

Dewey Decimal numbers for Other selected genealogical topics include:

Biography	920	Genealogy, heraldry	929.x
Bibliography	016.x	U.S. History, by date	973.x
Canada	971.x	Colonial 1607-1775	973.2
Church history	28x	Revolution 1775-1789	973.3
Europe	94x	Constitutional 1789-1809	973.4
Family histories	929.2	Civil War 1861-1865	973.7