Through the use of the direct solver, LINGO substitutes out all the fixed variables and constraints from the model. The remaining reduced set of constraints and variables are then classified as being either linear or nonlinear. LINGO's solver status window, which by default opens every time you solve a model, gives a count of the linear and nonlinear variables and constraints in a model. If any nonlinear variables or constraints are found in the model, the entire model is considered nonlinear and the relatively slower nonlinear solver must be invoked in place of the linear solver.