Common law is the body of law derived from court decisions which set a precedent and, hence, the resolutions contained in them are registered and will operate in subsequent cases.
In fact, under common law, if a new case arises which resembles a previous decision, the court will be bounded to solve it by using the previous ruling. On the contrary, if the court considers that the present dispute is different from prior ones and that it constitutes a "matter of first impression" then judges have to solve the issue without using precedents.
The North relied mostly on industry for income but the South was almost all agricultural. Without the slaves they didn't have many people left to work the land.