One consequence of the change in the user interface between versions 1 and 2 was a loss of certain features, and some flexibility in specifying input parameters. Examples include pipelined models and variable gas density. This sacrifice was deemed acceptable in the interests of streamlined and simplified execution, and because these features were not often used (as far as we know). In an effort to preserve these features in some form for dedicated users we also provide the capablility to call XSTAR as a fortran subroutine, which allows the user flexibility in specifying geometry, spectrum and density. This is described in Chapter 14.