In mathematics, a partially ordered set in order theory is chain complete if every chain in it has an upper bound.

Unlike complete posets, chain complete posets are relatively common. Examples include:

• Any complete poset
• The set of all linearly independent subsets of a vector space V, ordered by inclusion.
• The set of all partial functions on a set, ordered by

iff when

and

we have

and g | _A = f.

• If A is a collection of non-empty sets, the set of all partial choice functions on A, ordered as above.
• The set of all ideals of a ring, ordered by inclusion.
• The set of all consistent theories of a first order language.

Chain complete posets are interesting because of the Bourbaki-Witt theorem, and their connection with Zorn's Lemma.