Voice and tone are frequently used interchangeably in brand guidelines. They are not the same thing, and conflating them produces copy guidelines that are either too rigid to be useful or too vague to be followed.
Voice is who you are
Brand voice is the consistent personality that comes through in all communication. It does not change based on context. A brand that is direct, precise, and confident should sound that way whether writing a homepage headline or a support email.
Tone is how you feel in the moment
Tone adapts to context. The same brand voice can be warm in an onboarding email, serious in a legal notice, and energetic in a social post. The voice is consistent. The tone shifts to match the situation.
Building a tone matrix
A useful brand guideline includes a tone matrix: a table mapping communication contexts to the appropriate tone, what that means in practice, and what to avoid. This gives anyone writing for the brand a practical guide specific enough to be useful.