Double quotes in TeX

\lq{}\lq\lq\ = \rq\thinspace\rq\rq
\lq\lq\thinspace\lq\ = \rq\rq{}\rq\ = \rq\rq\rq

Run
  tftopl cmr10.tfm cmr10.pl
and in cmr10.pl see KRN and other stuff for characters '42, '47, '140, '134 to understand why spaces after different types of quotes are the way they are. (\lq\lq and \rq\rq form ligatures)

Run this to understand what those numbers mean:
\obeylines
\char'42
\char'47
...
\bye

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To understand how tfm files work, do this:

change something in cmr10.pl, for example in "(LABEL C v)", then generate new cmr10.tfm, then
use this test.tex:

\showboxbreadth \maxdimen
\showboxdepth \maxdimen
\setbox0=\hbox{va}
\showbox0
\bye

and see log file to check how those change worked