Postel's Law states that we should be as error-free as possible while also being tolerant and forgiving of errors. According to this principle, implementations should adhere as closely as possible to existing recommendations and standards without expecting other parties to follow this approach.

Origin

The term was coined by computer scientist Jonathan Postel in the 1981 specification of the Transmission Control Protocol (TCP); in RFC 793, he formulated the sentence "Be conservative in what you do, be liberal in what you accept from others".

  • Postel, J. (1981). Transmission Control Protocol. RFC 793. Internet Engineering Task Force (IETF).

Application in UX and UI design

In UX and UI design, Postel's Law can be applied in such a way that users should be offered information that is as unambiguous as possible and error-free functions, while at the same time user input should be processed as forgivingly as possible – that is, form fields, for example, should be designed in such a way that it is clear as quickly as possible what should be entered. If users do fill them out incorrectly (for example, swapping their first and last names), the system should be tolerant enough to accept and correct the error independently.

This strategy reduces barriers and moments of frustration, as users have to struggle with inflexible systems less often. Therefore, in the area of inclusion, taking into account Postel's Law is a real added value, since unintentional input errors due to motor or sensory impairments can be better processed, which greatly increases user-friendliness, especially for people with disabilities.

What is interesting about the principle is its universality. For example, we receive a chaotic email that is difficult to understand and contains unclear statements, or an email that is written in an inappropriate tone. How do we react in such cases? According to Postel's Law, you should suppress your first impulse – sarcastically write back, "Could it be any more complicated?" or insultingly bark back or "ghost" the other person. If you shout back in the same style as the person shouting in the forest, things will get more complicated and unsatisfactory for everyone involved. It is better to respond to a chaotic email in a structured and comprehensible way or to react to an offensive email in a friendly manner.

If we respond to stupidity with stupidity, to aggression with anger or to mistakes with ridicule, we make the situation worse. It would be better to respond to annoyance with composure, to destructiveness with a willingness to help or to indifference with passion.

Postel's Law, although actually devised for computer systems, is a deeply human principle.

Impact on the User Experience

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Further information