A musical notation symbol representing a natural note, also used in set theory to denote natural numbers.
Examples
Natural numbers notation
n \in \mathbb{N} \text{ where } n > 0 \text{ is a natural number}
Musical notation in mathematics
C\natural \text{ represents C natural in musical set theory}
Natural isomorphism notation
F(X) \stackrel{\natural}{\cong} G(X)