The Stacks project

Remark 4.29.3. Big $2$-categories. In many texts a $2$-category is allowed to have a class of objects (but hopefully a “class of classes” is not allowed). We will allow these “big” $2$-categories as well, but only in the following list of cases (to be updated as we go along):

  1. The $2$-category of categories $\textit{Cat}$.

  2. The $(2, 1)$-category of categories $\textit{Cat}$.

  3. The $2$-category of groupoids $\textit{Groupoids}$; this is a $(2, 1)$-category.

  4. The $2$-category of fibred categories over a fixed category.

  5. The $(2, 1)$-category of fibred categories over a fixed category.

See Definition 4.30.1. Note that in each case the class of objects of the $2$-category $\mathcal{C}$ is a proper class, but for all objects $x, y \in \mathop{\mathrm{Ob}}\nolimits (C)$ the category $\mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(x, y)$ is “small” (according to our conventions).

Comments (2)

Comment #5392 by Manuel Hoff on

I am a bit confused by the third and fourth item on the list. Aren't they the same?

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