Most brands have an intuitive sense of "how they talk", but very few have it written down anywhere that anyone drafting content on their behalf can refer to. That lack of a reference document is why, over time, different pieces of content from the same brand end up sounding like different brands.
What brand voice actually is
It's the set of concrete decisions about how your brand talks: whether it's informal or formal, whether it uses humour or keeps a more serious tone, which words it avoids and which it uses often, and what kind of sentences it prefers (short and direct, or more elaborate.
Why it's worth documenting
Without a reference document, everyone writing for the brand) an employee, an outside agency, a new team member — applies their own personal style, even while trying to imitate what already exists. Over time, those small accumulated differences make the brand lose consistency without anyone having consciously decided that.
What the document should include
It doesn't need to be long: a few examples of "do this" and "not this" sentences usually communicate far more than an abstract list of adjectives like "warm, professional, approachable". Including three or four words the brand would never use helps just as much as defining the ones it does.
How to keep it up to date
A brand's voice can evolve over time, especially if the target audience changes. Reviewing the document once a year, and updating it with real examples of recent content that worked well, keeps it from going stale and losing its usefulness.
Frequently asked questions
Who should write the brand voice document?
Ideally, someone who knows the brand well and has some writing sensibility, reviewing real examples of content that has already worked, rather than starting from scratch with abstract definitions.
Does a small business also need to document its tone of voice?
Yes, especially if more than one person writes content on its behalf — social media, blog, customer service. The sooner it's documented, the less work it takes to fix accumulated inconsistencies later.
Should tone of voice be exactly the same across every channel?
The underlying personality should stay the same, even as the format adapts: on social media it can be more informal and direct, in formal communications a bit more polished, without ever stopping sounding like the same brand.