Symbols

$$ \mathbb{N} \subset \mathbb{Z} \subset \mathbb{Q} \subset \mathbb{R} \subset \mathbb{C} $$

$\mathbb{N}$ is the set of natural numbers.

$\mathbb{Z}$ is the set of integers. $\mathbb{Z}$ stands for Zahlen, the plural of the german word Zahl.

$\mathbb{Q}$ is the set of rational numbers. $\mathbb{Q}$ stands for quoziente, the italian word for quotient.

$\mathbb{R}$ is the set of real numbers.

$\mathbb{C}$ is the set of complex numbers.