The author's attitude toward the subject matter and the reader - the personality of the prose itself.
Tone is the author's attitude as expressed through word choice, sentence structure, imagery, and narrative perspective. It's the personality behind the prose - the thing that makes you feel whether a writer is being sincere or sarcastic, warm or detached, playful or dead serious. Tone isn't what's being said; it's how it's being said. The same event described in a grim tone and a comic tone will produce entirely different reading experiences.
Tone is the first thing a reader senses, even before they grasp the plot or meet the characters. It tells them how to feel about what they're reading - whether to lean in with dread, settle in with comfort, or brace for sharp wit. Inconsistent or unintentional tone is one of the most common reasons manuscripts feel 'off' without the writer being able to identify why. Mastering tone means having full control over your reader's emotional relationship with your prose.
Adams's tone is dryly absurd - treating cosmic catastrophe with the casual irritation of a British commuter. The tone is the comedy; remove it and the plot is actually quite bleak.
McCarthy's spare, repetitive, almost liturgical tone creates a reading experience that feels like the barren world he's describing. The tone doesn't just describe the apocalypse - it enacts it.
If your prose shifts from lyrical to casual to formal within a single chapter without a clear reason, the reader will feel whiplash. Read your work aloud - tonal inconsistency is easier to hear than to see.
A comic tone for a devastating scene can work (it's called dark humor) but only if you're doing it deliberately. Accidental mismatches make readers distrust the narrator.
Tone is the writer's attitude (controlled by word choice and style). Mood is the reader's emotional response (created by setting, imagery, and pacing). They're related but distinct - you set the tone, the reader experiences the mood.
Take a single event - something simple like a character making breakfast - and write it three times in three different tones: once darkly humorous, once warmly nostalgic, and once cold and clinical. Don't change what happens. Only change how you describe it. Compare the three versions and identify the specific word choices, sentence lengths, and details that shifted the tone each time.