K&amp;R corresponds to C in 7th Edition Unix. The source code to v7 and other early Unix versions is browsable at: <a href="http:&#x2F;&#x2F;minnie.tuhs.org&#x2F;cgi-bin&#x2F;utree.pl" rel="nofollow">http:&#x2F;&#x2F;minnie.tuhs.org&#x2F;cgi-bin&#x2F;utree.pl</a><p>I find it interesting to see what C was like even earlier. Notably there was an incompatible break between the 6th and 7th editions, when the compound assignment operators were flipped. It used to be &quot;=+&quot; instead of &quot;+=&quot;. I guess Ritchie figured it&#x27;d be better to avoid ambiguity with expressions like &quot;x=-1;&quot;.