Guide to LaTeX (4th Edition) (Tools and Techniques

G
T E X Fonts
Computers work exclusively with numbers, or more precisely, only with
bits which may be interpreted as numbers. They do not know the difference
between the letter A and an apple, or that either even exists. For text
processing, all symbols, both input and output, need to be represented
as numbers somehow. The association between number and symbol is
called the encoding or layout. The latter term derives from the common
method of illustrating the encoding in the form of a table.
Encoding tables are by no means standard. The standard ascii scheme
is just one of several, and it is limited to 7 bits, or 128 characters. There
are 8-bit (256 characters) versions as well, in fact, a large number exist,
tailored to different computer systems and languages. The question of
coding the input for L AT E X documents with more than 7 bits are addressed
in Sections 2.5.9 and D.5.
In this appendix, we look at how the underlying T E X program deals
with output fonts, their nomenclature, and their encoding tables.
G.1 Font metrics and bitmaps
When T E X decides to output character nn in a particular font, all it needs
to know is how much room to leave for it. T E X does not care what the
character looks like, for that is the task of the DVI driver afterwards.
Information about the characters in each font are stored in various
files, all bearing the root name of the font but with different extensions.
This information is divided into files that contain only the sizes of each
symbol, and those with the actual drawing or image data.
.tfm T E X font metric files are the only font files read in by T E X itself.
They contain the sizes of the characters, such as width, height, and
depth. For slanted fonts, they also possess the ‘italic correction’
for each letter. Furthermore, they specify for which letter combinations
a different spacing is required, such as AV instead of AV,
487