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In paragraph copy write out numbers up to nine, use numeral for 10 and up. When using variable numbers in UI messaging to indicate user interaction it is best to stick with numerals to simplify translation requirements (ex: ‘4 license assignments remaining’).
Use exclamation points sparingly!
Use ‘Sentence case’ rather than 'Title Case' (there may be exceptions but this is the default). Buttons are not an exception.
Use the second person pronoun ‘Your’ to describe user specific tasks for generating content (ex: ‘Name your list').
Use ‘My’ to highlight content that benefits from a sense of ownership (ex: ‘My discussions' or 'My profile’).
Use an active voice instead of a passive voice.
In a sentence written in the active voice, the subject of sentence performs the action. In a sentence written in the passive voice the subject receives the action.
Active: The dog bit the man.
Passive: The man was bitten by the dog.
Be mindful of cultural differences (ex: customized messaging that uses an informal first name salutation is considered rude in some countries).
Confirm that translations represent the context of the action appropriately.
Where possible for messaging and UI interactions use language that falls at or below a seventh grade reading level.
For short microcopy avoid the use of punctuation. This is particularly true for modal titles, sub headings, and form input helper / error text.
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There are a number of great comprehensive company style guides out there we’ve only just scratched the surface. Here is some inspiration!
Check out some articles about the craft of UX writing.