C ommon law marriages do not result from couples living together. Living together has nothing to do with it. In a common law marriage, the parties perform a specific act to marry one another. They are then officially married without any ceremony. Common law marriages were permitted in Illinois until the 1920's.
Today there are only a few states that still allow common law marriages.
Historically, common law marriages were the only type of marriages until the latter part of the 19th Century. Back then, the vast majority of the people lived in remote areas and rarely came in touch with judges or clergy. As our population grew, a need to register marriages was apparently recognized which resulted in what we know today as ceremonial marriages.