# Definition:Cocone

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## Contents

## Definition

Let $\mathbf C$ be a metacategory.

Let $D: \mathbf J \to \mathbf C$ be a $\mathbf J$-diagram in $\mathbf C$.

A **cocone from $D$** comprises an object $C$ of $\mathbf C$, and a morphism:

- $c_j: D_j \to C$

for each object of $\mathbf J$, such that for each morphism $\alpha: i \to j$ of $\mathbf J$:

- $\begin{xy}\[email protected][email protected]+2px{ D_i \ar[r]^*+{D_\alpha} \ar[dr]_*+{c_i} & D_j \ar[d]^*+{c_j} \\ & C }\end{xy}$

is a commutative diagram.

## Also known as

Some authors, notably Saunders Mac Lane, dislike the name **cocone** and rather speak of **cones from the base $D$**.

Cones are then called cones *to* the base $D$.

So as to avoid the unavoidable ambiguity this gives rise to, on this web site, **cocone** is the designated term.

## Also see

## Sources

- 2010: Steve Awodey:
*Category Theory*(2nd ed.) ... (previous) ... (next): $\S 5.6$