principle. interface copy is a compression problem: the shortest string that stays true. labels name the thing, not the sentence; the explanation, if any, lives one layer down.

rule (law).

• lowercase for all app-authored copy. user-created names are never rewritten: a name the user typed keeps its case exactly, everywhere it renders.
• cut the subject when it is the interface itself; the app does not narrate itself in third person.
• one fact per line. two facts are two lines, or a line and a tooltip.
• buttons are verbs, two words at most. places are nouns and live in menus, not on buttons.
• state copy is a word, not a report: a label plus at most one short clause; details live behind the tooltip or expander.
• do not repeat the container: inside a surface titled by context, do not restate the name the user is already looking at.
• names are one word where one word holds (“absorb”, not “save what is on screen into this”). safety copy earns its words: a warning that prevents harm is never compressed past clarity.

worked example — compression, before and after.

before	after	rule
your changes have been saved successfully	saved	state is a word
are you sure you want to delete this item?	delete “morning setup”? its 2 automations go too	name the object and the blast radius
click here to open the full settings window	settings	places are nouns
an error occurred while applying your changes (code 87)	didn’t apply — the display went to sleep. wake it and try again	error voice: one clause, exact corrective action, no codes
no items have been created yet. get started by creating your first one!	nothing here yet — [create]	empty is a vessel: one line, one action, no cheerleading

localisation, the cheap-now layer (proposal, pending ratification). all copy goes through string keys from the start; the copy table is the key catalogue. no concatenated sentence fragments; plurals by rule, never by appending “s”. layouts must survive +40% text expansion, checked by a pseudo-locale screenshot in the harness. number and unit formatting is locale-aware for data; timestamps pick one clock convention and keep it. full translation and full right-to-left support are chapter-level commitments for later; expansion tolerance and keyed strings are not deferrable, because both are cheap now and expensive forever after.

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Canonical text: §5, voice, in the design language document.