Does anyone know why in books on category theory the notation for the subobject classifier is almost everywhere the capital greek letter $\Omega$? Gérard Lang

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Concepts and categories in perspectives], this was the notation used in "the initial IHES edition of SGA IV, where it is noted that the set $\Omega (X)$ of all subobjects of an object $X$ in a Grothendieck topos $\mathbb{E}$ defines a sheaf for the canonical topology on $\mathbb{E}$ and so, by Giraud's theorem, is representable by some object $\Omega$." That answers the question of where the convention originates, but not why... – Zhen Lin Feb 7 '13 at 17:55