I was browsing the TeXbook source file
texbook.tex from CTAN. In it, I came across the control word pair
\endtt (e.g., lines 445–447, looks like for marking up source code). But I couldn't find them mentioned in the TeXbook, nor are they in Knuth's
plain.tex macro file. I also tried
$ tex '\show\begintt' on the command line, which returns undefined.
I'm wondering, is
texbook.tex from CTAN the actual original file that Knuth authored? Because it just seems odd (to me, anyway) that he would have used any macros when writing his original book that he wouldn't have included as "plain TeX".