How “Orthogonality” Creates an “Intuitive” Interface

The literal meaning of orthogonal is “at right angles”. It’s an important part of any interface that qualifies as “intuitive”.

Imagine you’re filling out a table of operations and what they do. In my case, it was a keystroke-actions table. So Ctrl+X cuts text, and Alt+UpArrow moves a node in a hierarchical structure. Where does “cut structure” go?

The answer, of course, is “Alt+X”. But that’s easiest to see when you create a table, with CTRL, ALT, SHIFT and various combinations across the top, (including “None”), and various keys you can press running down the side.

Up Arrow   Move structure  
X  Delete text   here!  

When you do that, and assign keystrokes consistently, each row and column acquires a meaning. So “x” becomes “cut”, CTRL becomes “text”, and ALT becomes “Structure”. “Cut structure” is therefore naturally assigned to ALT+X.

That kind of arrangement makes an interface “intuitive”, because people can anticipate the effect of gestures they’ve never tried. You don’t even have to explain the meanings, necessarily. People will just come to understand what each gesture means, and attempt to put them together.

If their expectations are rewarded when they do, the interface qualifies as “intuitive”. Any gaps in the matrix make it “non-intuitive”, and any conflicts in meanings make it “anti-intuitive”.

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