Interestingly, the logic of mapping of two single quotes into a double quote was not built into TeX; rather, the metadata in each of the standard fonts simply used the ligature mechanism to handle it, just as they would specify that "f" followed by "t" should turn into the single-character "ft" ligature.
It's a shame that ASCII (and its visual representation on terminals from most manufacturers) has a wonderfully slanty left-quote character, but the right-quote character is typically shown as vertical. This makes TeX input files that contain quotations look kind of yucky, which is all the more galling as the original SAIL terminals had symmetric quotes, so everything looked beautiful there (never mind also having proper uparrow and downarrow characters to indicate superscript and subscript, rather than the ugly caret and underscore characters that we're all stuck with in ASCII-land TeX).
Source: Me, 1980 or so, working on TeX and Metafont.
It's a shame that ASCII (and its visual representation on terminals from most manufacturers) has a wonderfully slanty left-quote character, but the right-quote character is typically shown as vertical. This makes TeX input files that contain quotations look kind of yucky, which is all the more galling as the original SAIL terminals had symmetric quotes, so everything looked beautiful there (never mind also having proper uparrow and downarrow characters to indicate superscript and subscript, rather than the ugly caret and underscore characters that we're all stuck with in ASCII-land TeX).
Source: Me, 1980 or so, working on TeX and Metafont.