When writing a letter or mailing a package it's always best to know the exact address of the recipient to whom you are sending it to. When you do not know the address, but have another address where the person can be reached like their place of employment, their family or close friends, then you can address the package c/o--or "in care of" that third party.
Purpose of A C/O
Having a package or letter sent to someone who no longer lives at an address could potentially lead to the return of the item as undeliverable. The purpose of using c/o, which stands for "in care of," as part of an address is to provide a name that is attached to the address you are mailing to. In addition, it tells the postal customer who receives the item that it is not for them, but that they should give it to the other addressed recipient.
Using a c/o can be helpful in many ways. Two of those ways are, making sure packages and correspondence can be delivered to a person who has an address unknown to the sender and in helping some people stay organized. According to the United States Internal Revenue Service, some people receive important tax documents in care of their attorneys or accountants for use in their trusts, IRAs and other purposes.
Addressing the Envelope or Package
The proper way to address an envelope in care of another party is by placing the intended recipient's name at the top of the address block and, on the next line, write "c/o" followed by the c/o party's name. Under the c/o party's name follow with the c/o party's full address, as you would on a standard mailing address.