The "care of" line on a business envelope tells the postal service that the intended recipient lives or works at an address that belongs to someone else. It's a routing instruction, not an honorific, and formatting it incorrectly can delay mail or cause it to be returned. Most business writers encounter it rarely enough that they have to look it up every time. This guide covers the correct format, placement, and the situations where it applies.
What "care of" actually means
When you address a letter "care of" someone, you're telling the handler: this piece of mail belongs to Person A, but Person B at this address is responsible for passing it along. The abbreviation c/o (lowercase, with a forward slash) stands in for the full phrase in most business contexts. Both forms are correct. The abbreviated version is more common on envelopes because it's compact and widely understood by postal workers worldwide.
The phrase doesn't imply the recipient is a guest, a tenant, or anything else specific. It's a routing mechanism. A solicitor's office, a company's registered address, a forwarding agent, a hotel, a relative's home: all of these can appear after c/o without any implied social meaning.
Where to place the c/o line on a business envelope
The c/o line sits on the second line of the address block, directly below the recipient's name. The structure runs in this order:
- Line 1: recipient's full name
- Line 2: c/o [name of the person or organisation at the delivery address]
- Line 3: street address or building name
- Line 4: city, state or county, postcode
- Line 5: country (for international mail)
The postal service reads the address block from the bottom up to sort mail. The physical delivery point appears last, and the routing information (including c/o) sits above it. Swapping the order confuses automated sorting and can route the envelope to the wrong person entirely.
How to write it: format and capitalisation
Use lowercase c/o when writing the abbreviation on an envelope or in an inside address. Some older style guides show "C/O" in capitals, but the lowercase form is standard in current business correspondence in both British and American English. You don't need a colon after it. Don't write "c.o." with full stops; the slash is what makes the abbreviation recognisable.
A correctly formatted address block looks like this:
Dr. Sarah Ellison
c/o Hartley & Partners LLP
14 Cannon Street
London EC4N 6JJ
United Kingdom
If you're writing the full phrase rather than the abbreviation, use sentence case: "care of Hartley & Partners LLP." Don't capitalise "care of" mid-address unless it's the first word on its line and your house style capitalises every address line (some print systems do).
When to use a c/o line
The most common situations in business correspondence include:
Forwarding through a legal or registered agent. Companies often use a solicitor's address or a registered office as their official correspondence address. Sending a formal letter to a director via their firm's registered address requires a c/o line so the handler knows who inside the building should receive it.
Reaching a freelancer or consultant at a client's premises. If a contractor works on-site at your client's office, their mail needs a c/o line to separate it from the client's own post.
International correspondence through a forwarding agent. Some organisations use a local forwarding agent when they don't have a permanent address in a country. The c/o line names the agent; the physical address is the agent's premises.
Personal mail to a short-term address. This applies less often in strictly business contexts, but when a senior executive is temporarily stationed at a regional office, routing correspondence through that office's administrator is common practice.
Common mistakes to avoid
Putting c/o on the wrong line is the most frequent error. Writers sometimes place it after the street address, which makes the line read as a building description rather than a routing instruction. The c/o line belongs immediately below the recipient's name, not below the street.
Omitting the recipient's name entirely is another error. The whole point of the c/o notation is that there are two parties: the person the letter is for, and the person or organisation whose address it's going to. If you write only c/o [organisation] without naming the actual recipient above it, the handler has no one to pass the letter to.
Don't confuse a c/o line with an attention line. The attention line routes a letter to a specific person inside an organisation that is itself the recipient. The c/o line routes a letter through a third-party address to reach someone who isn't the address holder. The two serve different purposes and appear in different situations, though they both help direct mail to the right person.
Formatting the inside address of the letter itself follows the same rules as the envelope. If you use a c/o line on the envelope, carry it into the inside address too. Consistency between the two prevents confusion if the letter and envelope are separated after opening. For a complete guide to formatting the inside address on the letter itself, see how to write a business letter inside address.
A note on international addressing
The c/o convention is recognised in most countries and doesn't need translation. Universal Postal Union addressing guidelines accommodate forwarding notations, and postal operators in most countries treat c/o as a standard routing marker. When sending to countries with right-to-left scripts or non-Latin alphabets, confirm with your courier or postal provider whether the Latin abbreviation is sufficient or whether a transliterated local equivalent is expected.
One practical tip for international mail: place the country name on the final line in all capitals, regardless of your usual capitalisation convention. Sorting systems in transit countries use the last line to determine routing, and capitalised country names are more reliably machine-read.
Putting it together
The c/o line is a small piece of envelope formatting that does real work. Get the order right: recipient name first, c/o line second, physical address below that. Use lowercase c/o without a colon. Keep the recipient's name above the notation and the actual delivery address below it. Follow the same format in the inside address of the letter itself. That's the complete rule.