According to The Columbia Encyclopedia, Sixth Edition. 2001-07, "Civil law is based on written legal codes, a hallmark of the Roman legal system, in which disputes were settled by reference to a written legal code arrived at through legislation, edicts, and the like; common law is based on the precedents created by judicial decisions over time."
Louisiana is the only state in the United States to employ civil law. All 49 of the remaining states utilize common law, a form of law derived from England. The main difference behind this disparity is that England, and thus the English common law system, governed the early colonies inevitably becoming the United States of America. The Louisiana territory was first settled by French colonists, who later became subject to Spanish rule as a result of the French and Indian War. "In the secret Treaty of San Ildefonso (1800), Napoleon I forced the retrocession of the territory to France." Id.
After the Louisiana Purchase by the United States in 1803 , the Louisiana Code of 1825 was enacted. This code was largely based on the Napoleonic Code and some remaining concepts of Spanish civil law. The Louisiana Code has since been revised, but its base in civil law remains.