where it comes from
Louis Sullivan wrote the sentence in 1896, and Walter Gropius restated it for the school in the production principles of 1926: an object is determined by its nature, and to make it work you must first study what it is for. The chairs, lamps and teapots of the workshops were the proof. Nothing on them is there for any reason except the use.
what it means here
Every component starts from a sentence that says what it is for. The sentence picks the shape, the size, the color and the label, in that order. Usability is not a layer added to the form; it is the form. A button is a box with a verb because a hand needs a target and an eye needs to know what will happen.
the rules it produces
- buttons are 48 px tall and name an action
- messages say what happened, then what to do
- red is for the one thing to do
- feedback is a use: hover, focus and pressed are part of the form