Exercise 9, § 2

Tag 03KJ, permanent. Cite it as [Bourbaki, Tag 03KJ].

Theory of Sets, III, 223

An ordinal sum $\sum_{\iota \in \mathbf{I}} \mathbf{E}_\iota$ (§ 1, Exercise 3) is well-ordered if and only if I and each $\mathbf{E}_\iota$ is well-ordered.