All communication with the user in XSPEC is performed through the XSPEC parser, which itself sits on top of the Tcl user interface. The Tcl interface identifies the commands being given and provides a number of facilities including command recall, looping, and contingent control. The arguments to commands are interpreted by the XSPEC parser which extracts ranges, switches, and so on. This appendix summarizes the features of these parsers from the point of view of the user.