System Design Principles
Postel's Law
Be liberal in what you accept; conservative in what you send.
Takeaways
- 1
Accept common input variations without punishing users.
- 2
Trim whitespace and normalize formats behind the scenes.
- 3
Output only the information users need, clearly phrased.
Origins
Postel’s Law originated in network protocol design but applies directly to forms, search, and data entry. Users should not need perfect syntax to succeed, while the system should return clean, predictable responses.
Forgiving input paired with precise output builds trust.
Design Applications
- Support flexible date and phone formats.
- Use fuzzy search with helpful fallbacks.
- Write concise confirmation and error copy.
Frequently Asked Questions
What is the Postel's Law?
Be liberal in what you accept; conservative in what you send.
How do you apply Postel's Law in UI design?
Support flexible date and phone formats. Use fuzzy search with helpful fallbacks. Write concise confirmation and error copy.
Why does Postel's Law matter for user experience?
Accept common input variations without punishing users. Trim whitespace and normalize formats behind the scenes. Output only the information users need, clearly phrased.
