Answer:
In the United States before 1865, a slave state was a state in which the slave trade was legal, while a free state was one in which it was not. There were some enslaved persons in most free states in the 1840 census, and the Fugitive Slave Act of 1850 specifically stated that an enslaved person remained enslaved even when she or he fled to a free state. Between 1812 and 1850, it was considered important that the number of free and slave states were kept in balance, so new states were admitted in pairs.