Let $C$ be a category. A triangle of morphisms of $C$ consists of objects $X,Y,Z$ of $C$ and morphisms $f\colon X \to Y$, $g\colon Y \to Z$, and $h\colon X \to Z$. This is often pictured as a triangle

$\array {
X & \overset{f}\rightarrow & Y \\
& \searrow^{h} & \downarrow^{g} \\
& & Z
}$

A commutative triangle is determined entirely by $f$ and $g$; therefore, a commutative triangle is equivalent to a composable pair of morphisms.

Accordingly, one rarely hears of commutative triangles on their own; instead, the concept only comes up when one already has a triangle and asks whether it commutes. (This is different from the situation with commutative squares.)

Created on September 3, 2010 at 20:01:23.
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