Recognizability between mobile and desktop

How much can a design change when adapting it for mobile?

I go by this rule: the mobile and desktop versions should be mutually recognizable. If I’ve used a website on my computer and then open it on my phone, everything should be where I expect it to be — and vice versa.

If, say, there’s a row of six images on desktop and on mobile it becomes two rows of three — that’s fine. But if the images are replaced by a “View Photos” button that opens a popup — that’s not fine anymore. If there’s a large block of text on desktop and on mobile part of it becomes hidden with “Show more” — that’s fine. But if the text is edited down just for mobile — that’s not fine anymore.

I disagree with the idea of separating scenarios where people say things like: “On mobile, users are usually in a hurry, for them section X is more important than section Y, so let’s move it up”. That breaks the mutual recognizability.

You can put the mobile and desktop designs side by side and ask yourself: do they feel like two views of the same thing, just rearranged for screen size? Will someone who knows one version find their way around the other? If not — I’d ask for a redesign.