What you see is what you get
The classic trap: an invoice renders as an invoice, and editing it opens a wall of inputs that shares no layout with the thing being edited. The person has to map "the third input" onto "the line item total" and back again, and they have to do it every time.
The better shape is that editing happens on the object. Change a price on the line, add a line item in place, watch the total update where the total lives. One layout, one mental model, and the arithmetic is visible while you make it.
The same argument explains why clicking a calendar slot beats filling in a datetime field. The calendar already shows availability, adjacency and conflict. A form field discards all three and asks you to reconstruct them from memory.
Editing must be a visible state
Direct manipulation has one obligation: it must be obvious whether editing is currently on. If a value looks the same whether or not it is editable, people either poke at read-only text or type into something they thought was static.
An explicit affordance — a focus ring, a hover cue, an Edit toggle that visibly changes the surface — costs almost nothing and removes the entire class of confusion.
Consistency is a rule about repetition, not sameness
"Consistent" does not mean everything looks alike. It means:
- One object, one component. A customer looks like a customer wherever a customer appears.
- One surface per kind of object. If a record opens in a drawer from the list, it must not open in a modal from search. Three surfaces for one object is three mental models — see drawer vs modal vs page.
- One meaning per signal. A colour, an icon or a position that means two different things in two places has stopped carrying information.
- Same layer, same treatment. Two things that look different must be different. Where they are not, the difference is noise the reader has to discount.
Why it compounds
Consistency is the principle with the best return over time, because every repetition of a pattern is learned once and spent many times. It is also the one most easily lost, because losing it never breaks anything — each divergent screen works fine on its own.
Our own audit found the admin had six style families and twenty-four different
h1 treatments, none of which was a bug and all of which cost the reader. That
is what this principle costs when it is nobody's job.
The test
Pick one object. List every place it appears. If the list has more than one visual treatment or more than one editing surface, you have found the work.