Why are @, `, and $ not used in C?

[Jon H. LaBadie]

Sorry if I am treading old territory here.  My news feed was down for
a considerable time and I have just picked up this thread.

The compilers I have used, principally within the AT&T environment
have a vestigal error message relating to the back quote (`).

If one enters a character constant with the wrong quotes, eg. `a`
instead of 'a', the compiler complains about:

	Illegal BCD constant

I have tried numerous single and multi character combinations and
none were "legal BCD constansts".  I assume that BCD constants were
considered as a possible addition to the language at one time and
for whatever reason, rejected.

Does anyone have any insight into the history of this error message?

-- 
Jon LaBadie
{att, princeton, bcr}!jonlab!jon
{att, attmail, bcr}!auxnj!jon