What position variable in circular motion is analogous to the number line position variable in rectilinear motion?

Rectilinear motion and circular motion are similar in that, in their strict forms, they comprise single degree-of-freedom systems. As the position of a rectilinear movement can be noted with the location along the number lone, the position of a body undergoing circular motion is generally denoted by the angle (represented by the Greek letter theta) in either radians or degrees.