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## Font definition files

The description of how LATEX font attributes are turned into TEX fonts is usually kept in a font definition file (`.fd`). The file for family <family> in encoding <ENC> must be called `<enc><family>.fd`: for example, `ot1cmr.fd` for Computer Modern Roman with encoding `OT1` or `t1ptm.fd` for Adobe Times with encoding `T1`. Note that encoding names are converted to lowercase when used as part of file names. Whenever LATEX encounters an encoding/family combination that it does not know (e.g. if the document designer says `\fontfamily{ptm}\selectfont`) then LATEX attempts to load the appropriate `.fd` file. ``Not known'' means: there was no `\DeclareFontFamily` declaration issued for this encoding/family combination. If the `.fd` file could not be found, a warning is issued and font substitutions are made. The declarations in the font definition file are responsible for telling LATEX how to load fonts for that encoding/family combination.