Content

1. Voice and tone

Voice

Copy is direct, calm, helpful, and human. It is not chatty, apologetic, self-promotional, or passive.

table

Characteristic	Do	Don't
Direct	"Save changes"	"Go ahead and click Save to save your work"
Calm	"Connection lost. Check your network connection."	"Oops! Something went wrong."
Helpful	"No segments found. Adjust your search filters."	"No segments found."
Human	"This message will send to 4,200 people."	"This message will be delivered to 4,200 recipients."
Neutral	"The Agent is ready."	"You've unlocked a powerful new feature!"

Pronouns

Use "you" and "your" to address the person directly. Don't mix "my" and "your" on the same screen. Pick one and use it consistently throughout a flow. Remove possessives when they add no meaning.

Don't start a sentence with "this" or "that" unless the noun immediately follows. It's ambiguous in context and harder to parse.

Tone

Tone adjusts to what the person is experiencing. A person completing their first setup flow needs reassurance. A person facing a destructive action needs precision.

table

Context	Tone	Example
Task flow	Functional, invisible	"Save changes"
Onboarding / first use	Warm, clear	"You're all set. Your first message is ready to send."
Empty state	Helpful, inviting	"No segments yet. Create one to start targeting your audience."
Success	Brief, positive	"Campaign sent."
Warning	Clear, non-alarmist	"Deleting this attribute will remove it from 4,200 people."
Error	Direct, actionable	"Your changes couldn't be saved. Check your connection and try again."
Destructive / irreversible	Serious, precise	"Deleting this campaign is permanent and cannot be undone."
AI output	Outcome-first, hedged	"Based on recent behavior, these people are more likely to re-engage."

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2. Capitalization

Use sentence case everywhere (capitalize the first word and proper nouns only). That includes: headings, labels, buttons, tooltips, placeholder text, empty states, error messages, nav items, column headers.

When in doubt, use sentence case. Title case in labels creates inconsistency and adds visual noise: it's harder to scan, harder to localize, and creates a second capitalization system that readers have to learn.

Proper nouns

Always capitalize the following nouns:

• Customer.io

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3. Punctuation

Periods

Use periods for full sentences. Do not use periods for standalone labels, button text, nav items, or column headers. Periods for labels signal that something requires comprehension rather than a quick scan.

Exclamation points

Reserve exclamation points for genuine first-time milestones only to preserve impact. Exclamation points lose meaning through overuse. Never use exclamation points in errors, warnings, or routine confirmations.

Ellipses

Use ellipses only in progress and loading states.

There is one exception for buttons where it is appropriate to use when a button opens a dialog or requires additional input before the action completes. It signals "there's more to do before this happens" and removes hesitation from people who aren't sure what will be affected.

Em dashes and en dashes

Do not use em dashes (—). Em dashes have become strongly associated with AI-generated text and add visual noise in dense interfaces. Restructure the sentence or use a comma instead.

Use an en dash (–) for ranges.

Colons

Use colons to introduce a list or label-value pair. Do not add a space before the colon. Omit the colon when the label and value are visually separated by layout.

Ampersands

Use ampersands (&) only in established product names ("Data & Integrations") or space-constrained UI (nav items, table headers). Write and everywhere else.

Parentheses

Use parentheses to define an acronym or term on first use. Do not use to add asides or extra thoughts. Restructure the sentence instead.

Oxford comma

Always use an oxford comma in lists of three or more.

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4. Numbers, time, and dates

Numbers

• Use Arabic numerals always, including at the start of a sentence: 4,200 people not Four thousand two hundred people
• Format with commas: 1,200 not 1200
• Abbreviate when space is constrained, consistently within a view: 1.2K, 4.8M

Time

• 12-hour format, lowercase am/pm: 10:30 am, 4:00 pm
• Include time zone whenever relevant: 10:30 am ET
• Use an en dash for time ranges: 9:00 am – 5:00 pm

Dates

• Default: July 28, 2025
• Dense data tables only: MM/DD/YYYY

Relative time

Use relative time for timestamps and activity feeds. Use the appropriate unit once it reaches its threshold. Use 1 hour ago instead of 60 minutes ago.

Elapsed time:

• Just now
• 1 minute ago
• X minutes ago
• 1 hour ago
• X hours ago
• 1 day ago
• X days ago
• 1 month ago
• X months ago
• 1 year ago

Remaining time:

• X hours remaining
• X minutes remaining
• X seconds remaining
• About X minutes remaining (use "about" for estimates)

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5. Buttons and action labels

Rules

• Start with a verb.
• Keep it short and succinct: 1–3 words.
• Be specific enough that the button is self-explanatory without surrounding context.
• Cancel is always "Cancel", never "No", "Never mind", "Go back", or "Close".
• Use an ellipsis (…) when a button opens a dialog or requires additional input before the action completes. It signals "there's more to do before this happens" and removes hesitation from people who aren't sure what will be affected.

Generic verbs like "Submit", "Confirm", and "OK" require people to hold surrounding context to understand what will happen. Specific verbs like "Delete campaign" and "Send newsletter" let people act with confidence. This matters most for destructive actions, where a vague label can have significant consequences.

Types

table

Type	Pattern	Examples
Primary action	Verb + object	Create campaign, Add people, Send broadcast
Destructive	Delete + object	Delete segment
Confirmation	Verb restating the action	Delete campaign, Send now
Cancel	Cancel	Always Cancel
Save	Save / Save changes / Save and close	Match to interaction model
Progressive	Forward-motion verb	Next, Continue, Set up object
Dialog-triggering	Verb…	Export…
Loading state	Present participle…	Saving…, Sending…

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6. Onboarding

• Lead with the task, not background information.
• One instruction per step.
• Always show progress (step X of Y or equivalent).
• Confirm completion explicitly. Say what was set up and what the person can do now.

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7. Errors and system feedback

Error states are high-anxiety moments. Vague errors ("Something went wrong") make people feel helpless. Blame-framed errors ("You entered an invalid value") create frustration. A good error message turns a moment of friction into a moment of guidance. It is often the difference between a person recovering independently and submitting a support ticket.

Answer as many of these as possible:

1. What happened? State the problem plainly.
2. Why? Give context when it's honest and useful.
3. What now? Give a specific next step.

Validation errors (inline, adjacent to field)

Frame validation errors as instructions, not accusations.

System errors (something failed)

Permission errors

Payment and account errors

Use passive voice in payment, security, and account errors where active voice would imply blame or cause alarm.

Destructive warnings (before action)

State the consequence. If the action is reversible within a time window, say so. It's more informative and less alarming than stating only that something will be deleted.

Success messages

Confirm what happened. Keep it brief. No editorializing. Reserve positive, warm tone and exclamation points for genuine milestones only.

Rules

• Never use "Oops".
• Never blame the person: "You entered an invalid email" → "Enter a valid email address."
• Never be vague: "Something went wrong" is never acceptable.
• Never use exclamation points in errors.
• Always offer a recovery path when one exists.

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8. Inclusive language

Language can exclude people in ways that are both immediately obvious to the affected person and invisible to everyone else. For example, "easy" and "simple" invalidate the experience of people who find things hard.

Substitution

table

Don't use	Use instead
Whitelist	Allowlist
Blacklist	Blocklist
Master / slave	Primary / replica, leader / follower
Sanity check	Check, verify, review
Dummy data	Sample data, placeholder data
Kill / abort	Stop, cancel, end
Click	Select
See (as in "see all results")	View
Watch (video)	Play
Hit (a button)	Select
Easy / simple	Remove or rephrase
End-user	Person
Solution (as a noun)	Reframe as active verb phrase
Recalled / revoked / rescinded	Not available, not supported
He / she	They
He or she	They

Person-first

Use the most specific term available over generic abstractions.

No assumed context

• Always specify time zones when displaying times.
• Avoid idioms that don't translate: "low-hanging fruit", "hit the ground running", "ballpark".
• Use internationally unambiguous date formats when precision matters.
• Don't start a sentence with "this" or "that" unless the noun immediately follows. It's ambiguous and difficult to translate.

No urgency or pressure language

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9. Accessibility and alt text

Alt text makes images accessible to people who use screen readers and provides context when images don't load. Every informative image needs alt text. Decorative images should use alt="" to hide them from screen readers.

Rules

• Write alt text for all informative images.
• Use alt="" for decorative images.
• Do not start alt text with "Image of". Screen readers already announce it's an image.
• Keep alt text under 140 characters.
• Describe meaning and context, not visual detail.
• Do not repeat the caption in alt text.
• Use the same word choices in alt text as in nearby body text and captions.

Alt text in Customer.io contexts

• Empty state illustrations: If the heading and body copy already convey the message, the illustration is decorative. Use alt="".
• Charts and data visualizations: Describe the key insight, not the structure. "Open rates peaked at 42% in March, driven by the re-engagement campaign." For complex charts, note the general pattern and link to the underlying data.

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10. AI-specific copy

AI features carry a specific set of trust risks that standard copy rules don't cover. Overclaiming ("optimal", "best") sets expectations the system can't always meet. Describing the technology instead of the outcome makes copy feel like a press release. Presenting AI output as final removes agency. Calling the Agent "a bot" or "a tool" undersells a capability that is core to the platform's value.

Rules

• Lead with the outcome, not the AI. Never use "AI-powered" as a descriptor.
• AI output is always a suggestion until a person acts on it. Never present it as final or authoritative.
• Use hedging language for probabilistic recommendations: "likely", "based on", "suggested".
• Describe what the AI is doing in plain terms, not the technology behind it.
• The AI Agent is "the Agent". Never "the bot", "the AI", or "the tool".
• When the Agent completes a task, confirm what it did and what the person should review.

Outcome-first, not AI-first

Confidence and uncertainty

AI in progress

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