A warning message is generated because no condition is given for one of the boundaries:

The error message is triggered because is being used as the temporal variable. This can seen because is identified as the independent variable in the error message. One way to specify a unique solution is by adding a second boundary condition for at :

If is really the temporal-independent variable, then two initial conditions are needed at since the equation is second order in . The original example only has one condition at . Including an initial condition on the first derivative in the direction at resolves the error message:

When the method of lines is used to solve a PDE, a temporal variable must be chosen. This defines the direction of integration. The number of boundary conditions and initial conditions can be seen from the equation. For example, consider . If is the temporal variable, is the direction of integration, and the equation is first order in in that direction. Therefore, one initial condition is needed at , and one boundary condition is needed at each boundary and . If is the temporal variable, is the direction of integration, and the equation is second order in in that direction. Thus, two initial conditions are needed at , and one boundary condition is needed at or :