Alternatively, since the natural numbers naturally form a subset of the integers (often denoted Z {\displaystyle \mathbb {Z} } ), they may be referred to as the positive, or the non-negative integers, respectively.[30] To be unambiguous about whether 0 is included or not, sometimes a subscript (or superscript) "0" is added in the former case, and a superscript "*" is added in the latter case:[3]