A common abbreviation on letters is "c/o," which means "in care of." This is used in addresses to indicate that the name listed on the first line of the "to" address block is either not a regular recipient of mail at the address or that the name might not be familiar to others who handle the letter. The name used on the same line as the "c/o" can be that of another person or a business at that address. Formatting an address that has a "c/o" line is straightforward.
Write the recipient's name on the top line of the "to" address space of the envelope or package.
Write "c/o" and the name of the person or business with whom the recipient is associated. For example, if you're sending a letter to your friend who is staying at her cousin Jane Doe's house for a couple of weeks, you'd write, "c/o Jane Doe." If the recipient works at a business that is located in a center with several other businesses -- for this example, pretend the name of the business is Generic Hatmaking -- you would write "c/o Generic Hatmaking."
Fill out the rest of the address as usual, with the street address or post office box, any suite or apartment numbers, the city and state, and the ZIP code.
Write "c/o" using lowercase letters.
You might see "c/o" handwritten diagonally on older letters as if the abbreviation were in a square, with the "c" in the upper left corner and the "o" in the lower right corner. You can write it this way if you prefer.
You can use "c/o" in the sender's address space if you're sending the letter from someplace where you don't normally receive mail, such as from a vacation spot.
When in doubt, use "c/o." If you know the recipient normally receives mail at the address, you don't have to use "c/o" with either residential or commercial addresses. But if you're not sure, "c/o" will show the postal carrier that even if she doesn't recognize the recipient's name, the letter should go to that address. Making sure addresses are written legibly and clearly is crucial so the post office staff can read the address and get the mail to the intended recipient. A messy or unclear address may make it difficult for the post office to read or send the mail. If that situation occurs, the mail will be sent back to the original sender.