Initial Segment of Ordinals under Lexicographic Order

Theorem
Let $\operatorname{Le}$ denote the lexicographic order for ordinals.

Then, the initial segment of $(1,\varnothing)$ is a proper class.

Proof
Define the function $F$ such that $F(x) = (\varnothing,x)$. Then, $F:\operatorname{On} \to (\operatorname{On} \times \operatorname{On})_{(1,\varnothing)}$, since $\varnothing < 1$.

By Equality of Ordered Pairs, $F$ is injective. But since $\operatorname{On}$ is a proper class by the Burali-Forti Paradox, the initial segment of $(1,\varnothing)$ is a proper class as well.

Source

 * : $\S 7.54 \ (2)$