The locale-dependent prolog file is /usr/openwin/lib/locale/$LANG/print/prolog.ps and it usually is a PostScript file. This file can contain included Type1 fonts that define PostScript prolog information with some additional PostScript routines. One of the main goals of the prolog file is to set the locale's fonts in an alias for a set of font names that are pre-defined and used in the mp(1).
Support for this file is provided to conform with the prolog.ps file that is used by /usr/bin/mp. If this file exists, then it is given preference and mp.conf file is not scanned for backward compatibility.
The sections about mp.conf file that follow are reprinted from the OpenWindowsTM Localization Guide.