A compact symmetric object (CSO) is a radio-loud galaxy characterized by symmetrical jets on either side of an active central engine, usually assumed to be an active galactic nucleus (AGN). The jets are distinguished by their spatial size scales, with lengths of less than 1 kpc. Classical double radio sources, by contrast, typically have radio jets many kpc or Mpc in length. It has been suggested that CSOs represent the earliest stages of an AGN evolving into an active FR II radio galaxy.