Textbook of Usability

Jakob's Law, formulated by Jakob Nielsen, states: "Users spend most of their time on other sites. This means that users prefer your site to work the same way as all the other sites they already know."

The law captures a practical consequence of mental models: users do not encounter your interface in isolation. They arrive with expectations formed by hundreds of other sites and apps they use daily. A cart icon means "shopping basket" because thousands of sites have used it that way. A hamburger menu collapses navigation because the convention is now universal. A down arrow next to a label means a dropdown.

Violating conventions costs usability:

• Users must learn unfamiliar patterns
• They make mistakes based on their prior expectations
• They distrust the unfamiliar — novel interaction patterns feel suspicious
• Discoverability suffers when users can't predict where things should be

Jakob's Law is the psychological basis for design conventions and platform consistency. Apple's Human Interface Guidelines, Google's Material Design, and Microsoft's Fluent Design all codify conventions so that apps on each platform feel consistent. Breaking conventions should be done only when the benefit clearly outweighs the cost of confusion.

The law is related to Nielsen's "consistency and standards" heuristic but is broader: it extends beyond internal consistency to consistency with the entire ecosystem of interfaces users have learned. Innovation in interaction design is possible but expensive — radical departures must justify the learning cost.