What is the main function of an International Reply Coupon (IRC)?

The main function of an International Reply Coupon (IRC) is to prepay for a reply. IRCs are designed to facilitate international correspondence by allowing the recipient to obtain postage for replying to the sender. When a sender includes an IRC in their outgoing mail, it acts as a form of prepaid postage that the recipient can exchange for stamps to send a response back to the original sender, thus eliminating the need for the recipient to purchase postage on their own.

This process is particularly useful in international communication, where varying postal rates could make it cumbersome for the recipient to reply. By providing an IRC, the sender proactively covers the cost of the reply, ensuring that the recipient can respond without additional expenses.