Informally, a diagram in a category CC consists of some objects of CC connected by some morphisms of CC. Frequently when doing category theory, we “draw diagrams” such as

A f B h k C g D\array{A & \overset{f}{\to} & B\\ ^h\downarrow && \downarrow^k\\ C& \underset{g}{\to} & D}

by drawing some objects (or dots labeled by objects) connected by arrows labeled by morphisms.

There are two natural ways to give the notion of “diagram” a formal definition. One is to say that a diagram is a functor, usually one whose domain is a (very) small category. This level of generality is sometimes convenient.

On the other hand, a more direct representation of what we draw on the page, when we “draw a diagram,” only involves labeling the vertices and edges of a directed graph (or quiver) by objects and morphisms of the category. This sort of diagram can be identified with a functor whose domain is a free category, and this is the most common context when we talk about diagrams “commuting.”


Let CC be a category.

Diagrams shaped like categories


If JJ is a category, then a diagram in CC of shape JJ is simply a functor D:JCD\colon J \to C.

This terminology is often used when speaking about limits and colimits; that is, we speak about “the limit or colimit of a diagram.” Similarly, it is common to call the functor category C JC^J the “category of diagrams in CC of shape JJ”.

Diagrams shaped like graphs


If JJ is a quiver, then a diagram in CC of shape JJ is a functor D:F(J)CD\colon F(J) \to C, or equivalently a graph morphism D¯:JU(C)\bar{D}\colon J \to U(C).

Here F:QuivCatF\colon Quiv \to Cat denotes the free category on a quiver and U:CatQuivU\colon Cat \to Quiv the underlying quiver of a category, which form a pair of adjoint functors. These are the sorts of diagrams which we “draw on a page” — we draw a quiver, and then label its vertices with objects of CC and its edges with morphisms in CC, thereby forming a graph morphism JU(C)J\to U(C).


  • For either sort of diagram, JJ may be called the shape, scheme, or index category or graph.

  • Note that given a diagram D:JCD:J\to C, the image of the shape JJ is not necessarily a subcategory of CC, even if JJ is itself taken to be a category. This is because the functor DD could identify objects of JJ, thereby producing new potential composites which do not exist in JJ. (Sometimes one talks about the “image” of a functor as a subcategory, but this really means the subcategory generated by the image in the literal objects-and-morphisms sense.)

  • CC must be a strict category to make sense of U(C)U(C); however, F(J)F(J) always makes sense.

Commutative diagrams

If JJ is a category, then a diagram JCJ\to C is commutative if it factors through a thin category. Equivalently, a diagram of shape JJ commutes iff any two morphisms in CC that are assigned to any pair of parallel morphisms in JJ (i.e., with same source and target in JJ) are equal.

If JJ is a quiver, as is more common when we speak about “commutative” diagrams, then a diagram of shape JJ commutes if the functor F(J)CF(J) \to C factors through a thin category. Equivalently, this means that given any two parallel paths of arbitrary finite length (including zero) in JJ, their images in CC have equal composites.


  • The shape of the empty diagram is the initial category with no object and no morphism.

    Every category CC admits a unique diagram whose shape is the empty (initial) category, which is called the empty diagram in CC.

  • The shape of the terminal diagram is the terminal category J={*}J = \{*\} consisting of a single object and a single morphism (the identity morphism on that object).

    Specifying a diagram in CC whose shape is {*}\{*\} is the same as specifying a single object of CC, the image of the unique object of 11. (See global element)

  • A diagram of the shape {ab}\{a \to b\} in CC is the choice of any one morphism D ab:X aX bD_{a b} : X_a \to X_b in CC.

    Notice that strictly speaking this counts as a commuting diagram , but is a degenerate case of a commuting diagram, since there is only a single morphism involved, which is necessarily equal to itself.

  • If JJ is the quiver with one object aa and one endo-edge aaa\to a, then a diagram of shape JJ in CC consists of a single endomorphism in CC. Since aaa\to a and the zero-length path are parallel in JJ, such a diagram only commutes if the endomorphism is an identity. Note, in particular, that a single endomorphism can be considered as a diagram with more than one shape (this one and the previous one), and that whether this diagram “commutes” depends on the chosen shape.

  • A diagram of shape the poset indicated by

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow && \downarrow \\ b' &\to& c } \right\}

    is a commuting square in CC: this is a choice of four (not necessarily distinct!) objects X a,X b,X b,X cX_a, X_b, X_{b'}, X_c in C, together with a choice of (not necessarily distinct) four morphisms D ab:X aX bD_{a b} : X_a \to X_b, D bc:X bX cD_{b c} : X_b \to X_c and D ab:X aX bD_{a b'} : X_a \to X_{b'}, D bc:X bX cD_{b' c} : X_{b'} \to X_c in CC, such that the composite morphism D bcD abD_{b c}\circ D_{a b} equals the composite D bcD abD_{b' c}\circ D_{a b'}.

    One typically “draws the diagram” as

    X a D ab X b D ab D bc X b D bc X c \array{ X_a &\stackrel{D_{a b}}{\to}& X_b \\ {}^{\mathllap{D_{a b'}}}\downarrow && \downarrow^{\mathrlap{D_{b c}}} \\ X_{b'} &\stackrel{D_{b' c}}{\to}& X_{c} }

    in CC and says that the diagram commutes if the above equality of composite morphisms holds.

    Notice that the original poset had, necessarily, a morphism aca \to c and could have equivalently been depicted as

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow &\searrow& \downarrow \\ b' &\to& c } \right\}

    in which case we could more explicitly draw its image in CC as

    X a D ab X b D ab =D bcD abD bcD ab D bc X b D bc X c \array{ X_a &\stackrel{D_{a b}}{\to}& X_b \\ {}^{\mathllap{D_{a b'}}}\downarrow &\searrow^{\stackrel{D_{b c}\circ D_{a b}}{= D_{b' c}\circ D_{a b'}}}& \downarrow^{\mathrlap{D_{b c}}} \\ X_{b'} &\stackrel{D_{b' c}}{\to}& X_{c} }
  • By contrast, a diagram whose shape is the quiver

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow && \downarrow \\ b' &\to& c } \right\}

    is a not-necessarily-commuting square. The free category on this quiver differs from the poset in the previous example by having two morphisms aca\to c, one given by the composite abca\to b\to c and the other by the composite abca \to b'\to c. But the poset in the previous category is the poset reflection of this F(J)F(J), so a diagram of this shape commutes, in the sense defined above, iff it is a commuting square in the usual sense.

  • A pair of objects is a diagram whose shape is a discrete category with two objects.

  • A pair of parallel morphisms is a diagram whose shape is a category J={ab}J = \{a \stackrel{\to}{\to} b\} with two objects and two morphisms from one to the other.

    Notice that if we required {ab}\{a \stackrel{\to}{\to} b\} to be a poset this would necessarily make these two morphisms equal, and hence reduce this example to the one where J={ab}J = \{a \to b\}. In other words, a diagram of this shape only commutes if the two morphisms are equal.

  • A span is a diagram whose shape is a category with just three objects and single morphisms from one of the objects to the other two;

    J={ a b c} J = \left\{ \array{ && a \\ & \swarrow && \searrow \\ b &&&& c } \right\}

    dually, a cospan is a diagram whose shape is opposite to the shape of a span.

    J={b c a} J = \left\{ \array{ b &&&& c \\ & \searrow && \swarrow \\ && a } \right\}
  • A transfinite composition diagram is one of the shape the poset indicated by

    J={a 0 a 1 b}, J = \left\{ \array{ a_0 &\to& a_1 &\to& \cdots \\ & \searrow & \downarrow & \swarrow & \cdots \\ && b } \right\} \,,

    where the indices may range over the natural numbers or even some more general ordinal number.

    This is a non-finite commuting diagram.

  • tower diagram

Revised on January 23, 2017 09:00:45 by Urs Schreiber (