Longitudinal tension in a web span depends on several factors including the incoming web speed, the outgoing web speed, the current value of the web tension, the previous span tension, and the span length. In an effort to emphasize the importance of first principles modeling of a web line using primitive elements, this paper focuses on the variable span length effects on the longitudinal tension using a nonlinear span model. A varying span length may occur as a result of non-circular unwind and idle rollers, as well as eccentric rollers. The entry section of a typical roll-to-roll web line will be is modeled using both linear and nonlinear span models. Comparisons are made of predicted and measured tension results to illustrate when the nonlinear model should be used. It is demonstrated that the nonlinear model is useful in studying disturbance rejection in a web line with and without a dancer following the unwind roll.