Before you even think about configuring the font declarations by producing a file |fontmath.cfg| or |fonttext.cfg|, you should read the documented file |fontdef.dtx|. This is the source file from which the default files |fonttext.ltx| and |fontmath.ltx| are produced; it contains information concerning the contents of the default files and what sort of customisation is possible. In particular, it describes in detail the effects of individual customisations on document portability including: which customisations can be made without endangering the ability to exchange documents with other sites (even if the formatting differs); and which things should be left untouched because they will make your system so different from others that the documents it produces will be non-portable.