This sounded a bit far-fetched so I did some digging. It is true that C89 had a maximum of 6 characters for external identifiers. C89 is often called ANSI C.<p>C89 says:<p>&quot;The implementation shall treat at least the first 31 characters of
an internal name (a macro name or an identifier that does not have
external linkage) as significant. Corresponding lower-case and
upper-case letters are different. The implementation may further
restrict the significance of an external name (an identifier that has
external linkage) to six characters and may ignore distinctions of
alphabetical case for such names.&#x2F;10&#x2F; These limitations on identifiers
are all implementation-defined.&quot;<p>Source: <a href="http:&#x2F;&#x2F;flash-gordon.me.uk&#x2F;ansi.c.txt" rel="nofollow">http:&#x2F;&#x2F;flash-gordon.me.uk&#x2F;ansi.c.txt</a>