People expect your product to work like the ones they already use.
Users spend most of their time on other sites, so they expect yours to work the same way as the ones they already know.
Jakob Nielsen, the usability researcher behind the 10 usability heuristics, put the law into words in 2000: users spend most of their time on other sites, so they prefer your site to work the same way as all the other sites they already know. The reason is mental models. People build a general picture of how websites work from the ones they use most, and bring it to every new site. When a design matches that picture, it feels easy. When it breaks it, people slow down and make mistakes. Nielsen has since said the law applies to apps and anywhere else people use many different designs.
A shop page is empty. Place the logo, search, cart and menu where you would look for them, then see where most shops put them. Your guesses come from every shop you have used before.
Put common things where people already expect them, and save new ideas for the parts that make your product different.
Open almost any online shop and the cart sits in the top-right corner, as a bag or a trolley with a count of the items in it. A shop that moves it to the bottom-left behind a custom icon may look distinctive, but every visitor now has to search for it at the moment they want to pay. Put it where people already look, and they find it without thinking. The same goes for the logo that links home, search at the top, and underlined links.
When the familiar pattern is genuinely worse and the gain is big enough to repay the learning, change it, but test it with users first and make the new way easy to discover. Pull-to-refresh and swipe gestures were new once. Also look at whose expectations you are meeting: an expert tool should follow the conventions of its own field, not of consumer apps, and people in different countries can expect different patterns.
Ask someone who has never seen the product to find the cart, search and sign-in without help. If they hesitate on any of them, that part is not where they expect it.
Jakob's Law says users spend most of their time on other sites, so they prefer your site to work the same way as the sites they already know. In practice: put common things where people expect them.
Jakob Nielsen, a Danish usability researcher and co-founder of the Nielsen Norman Group, formulated it in 2000. He also created the 10 usability heuristics.
The shopping cart in the top-right corner of an online shop. People look for it there because nearly every shop puts it there, so a cart moved elsewhere gets missed.
No. Keep the common parts familiar, such as navigation, search and checkout, and be original where it makes your product better. If you change a convention, test it with users first.
People build a mental model of how websites and apps work from the ones they use most. Jakob's Law follows from that: a design that matches the model feels easy, and one that breaks it feels hard.
A clothing shop's new app moved the cart to the bottom-left corner and gave it a custom icon so the app would look different. Since the redesign, support gets messages asking where the cart is, and fewer people reach checkout.