# Opening piece

**The beginning** is a term from set theory and order theory.

## definition

A class is called the beginning of the ordered class if .

Every ordered class is divided into two disjoint subclasses:, where .

## See also

## literature

- F. Hausdorff:
*Fundamentals of a theory of ordered sets .*In:*Mathematical Annals.*65. 1908, p. 439