Postcards generally come in two formats: horizontal and vertical. There are also irregularly shaped postcards.
Horizontal postcards are divided into two sections: the left section for the text and the right section for the recipient's address, name, and phone number. The sender's information can also be included for return in case of delivery failure.
Vertical postcards are divided into two sections: the top section for the text and the bottom section for the recipient's address, name, and phone number.
Irregularly shaped postcards don't have a fixed format, but you should arrange the text and recipient's address information neatly to facilitate postal staff reading, entering, and delivery.
Regarding postcodes, regardless of whether it's horizontal, vertical, or irregularly shaped, the standard format is to fill in the recipient's postcode in the six boxes in the upper left corner and the sender's postcode in the box in the lower right corner. Many postcards have now removed the lower right postcode section; even without the boxes, you can still use this format, depending on your preference.