What's the relationship between vfs and tfms?

[Rebecca and Rowland <rebecca@astrid.u-net.com>]

Thanks to the helpful replies I got to my last query on this list, I find
myself confused by something else.

It's this tfm and vf business.  I gather that TeX uses .tfm files as its
source for fount metric information while typesetting, and that .vf files
are used by the .dvi driver to work out which glyphs in the `real' fount
should be used.  (this all makes my head hurt because the `real' fount is
usually an 8r encoded not at all real fount really that needs to be
re-encoded by the dvi driver, but I can handle that if I don't think about
it at the same time as thinking about .vfs and TeX and things)

Now then, I've got a fount called fkar8a that I've installed.  It's been
re-encoded by fontinst to 8r encoding, so I've got fkar8r.tfm.

This 8r encoded fount was used by fontinst (when it was a .pl and .mtx
file) to create fkar8t.vpl.

So far, so good.  Now then, if I run pltotf on fkar8r.pl, I get fkar8r.tfm.
This doesn't make my brain overheat.

If I run vptovf on fkar8t.vpl, I get fkar8t.vf and fkar8t.tfm:

This is VPtoVF, version 1.4
Input file: fkar8t.vpl
Output TFM file: fkar8t.tfm
Output VF file: fkar8t.vf

Obviously, TeX uses fkar8t.tfm for typesetting.  But how does anything know
to use the .vf file?  And what use exactly is the .vf file put to?

I could guess that either there's a flag in the .tfm file that tells the
dvi driver to look for a .vf file, or that the dvi driver looks for a
`real' fount called fkar8t (either fkar8t.pk, fkar8t.mf, or an entry in a
mapping table such as OzTeX's config file or DVIPS's psfonts.map), and
failing to find one, looks for a .vf file corresponding to it.

As for exactly what's done with the .vf file, I'm not quite sure.

Could someone cast some light?

And... (this is where my brain *does* overheat, melt down, and explode),
when I run vftovp on fkar8t.vf, this happens:

Input file: fkar8t.vf
Reading TFM file: :TeX-fonts:New tfms:fkar8t.tfm
Reading TFM file: :TeX-fonts:New tfms:fkar8r.tfm
Check sum in VF file being replaced by TFM check sum
Output file: fkar8t.vpl

Why are two .tfms being read?  Can anyone explain what's happening here?

Thanks in advance,
Rowland.

P.S.  At this rate, I suspect finishing any fontinst documentation will
take a little longer than I was expecting, but I seem to be making some
progress.