Postel's law

proper noun

The design principle that software should be conservative as to its own behaviour (i.e. strictly compliant with specifications) but liberal in the behaviour it tolerates from other systems.

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Meanings

proper noun

alternative Postel's Law

  1. computingThe design principle that software should be conservative as to its own behaviour (i.e. strictly compliant with specifications) but liberal in the behaviour it tolerates from other systems.

    Contravariance is consistent with Postel's Law: “Be liberal in what you accept, and conservative in what you send.”

    2005, Jesse Liberty, Visual C# 2005: A Developer's Notebook, O'Reilly Media, →ISBN, page 47

Origin

Named after Internet pioneer Jon Postel (1943–1998).

Full word history & related roots →
Cite this entry
APA

Postel's law. (n.d.). In WatkinsX Dictionary. Retrieved November 4, 2026, from https://dictionary.watkinsx.com/word/Postel's%20law

MLA

“Postel's law.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/Postel's%20law. Accessed November 4, 2026.

Chicago

WatkinsX Dictionary, s.v. “Postel's law,” accessed November 4, 2026, https://dictionary.watkinsx.com/word/Postel's%20law.

Source: Postel's law on Wiktionary, CC BY-SA 4.0.