Dotted circle

In Unicode, the dotted circle (◌) is a non-significant typographic character used to illustrate the effect of a combining mark, such as a diacritic mark. It can also be used to indicate a spot where a character ought to be, but it is seldom used for anything else.

Illustration
A Unicode combining mark combines with a preceding character. When used as stand-alone, it would combine unintentionally with a preceding character (possibly a space):
 * Diacritic ̒ used alone between regular spaces
 * Diacritic ◌̒ used after a character

Using the generic dotted circle character also shows the relative positioning of the diacritic.