Voice and tone

Voice is the stable baseline, tone is the dial. Voice is the vocabulary, rhythm, and attitude that make copy recognizably yours across every surface. It should hold steady from screen to screen, though it does evolve as the brand does, usually on a rebrand-sized timescale rather than a per-message one. Tone is what you adjust for the situation, the audience, and the emotional state they are in when they hit the words. Mailchimp's public style guide made the split famous with the obvious case: the same brand can be playful in an empty state and completely straight in a billing failure.

Writing it down is what makes it usable. The format that survives real teams: three to five voice traits, each with a 'we do this / we do not do this' pair, plus a tone matrix mapping situations (celebration, error, security warning, marketing page) to the register each gets. Adjectives alone ('friendly, human, bold') are useless, because every brand on earth claims all three.

Gotcha: tone failures cluster exactly where the user's emotional state runs highest. A joke in an error, an exclamation mark on a declined card, or 'Oops!' after someone loses work reads as the product not taking them seriously. When in doubt, drop the personality and be clear.