IBM 1620 invalid character symbol

[Ian Clifton via Unicode]

William Overington wrote:

> A digit with a bar over the top is used to express the common logarithm of a number that is both greater
> than zero and also less than one.

 > https://en.wikipedia.org/wiki/Common_logarithm


Gosh, I’d forgotten that usage, although I now remember being taught it at school.

Another use of over bar for negative numbers is in crystallography—I believe the motivation is that it’s useful
to have a very compact notation for (often space separated triples of) small integers of either sign.
-- 

Ian Clifton