Postcards come in many forms, including single postcards and double postcards. It is folded in half by connecting two single pieces. Folding forms include up and down folding and left and right folding. When in use, one of them is addressed to the sender, and the other receipt (copy) is addressed with its own address, which is folded and sent. When the recipient replies, just write it on the supplementary page.
Some two-piece postcards are printed with stamp maps, and postage-free two-piece postcards should be stamped on the front and back respectively by the sender. Two postcards can be used for domestic and international communication.
Extended data:
The address and name of the recipient are written on the right side of the postcard (on the painted column).
The sender's address is also written on the right, after the recipient's address and name, and then on a new line, write the sender's address and name (if you use a private mailbox, don't write your name, if it is a public mailbox, it is best to write the recipient's postal code in the upper left corner and your postal code (of the postcard sender) in the lower right corner.
The sender's address can be written or not. If you are quite sure that the recipient's address is correct, you can leave it blank. If you are not sure, it is better to write it down for return.
If the words are written in the left margin, you don't have to write "respect and salute". If you and the recipient are close friends, you can write it like this, or you can use "I wish you good health" or "I wish you all the best".
References:
Baidu Encyclopedia-International Standardized Postcard Size