Full Idea
Every member of an ordinal is itself an ordinal, and every ordinal is a transitive set (its members are also its subsets; a member of a member of an ordinal is also a member of the ordinal).
Gist of Idea
Members of ordinals are ordinals, and also subsets of ordinals
Source
Michal Walicki (Introduction to Mathematical Logic [2012], 2.3)
Book Reference
Walicki,Michal: 'Introduction to Mathematical Logic' [World Scientific 2012], p.89