— Vex & Rebel, founders
The address side of the card must be divided into an upper portion and a lower portion, with or without a horizontal rule. The portion of the address side that does not contain the destination address is the message area.
The address, postage, and any Postal Service marking or endorsement must appear within the portion containing the destination address. As an alternative, when the destination address appears within the lower portion, it is permissible for the postage, return address, and any USPS marking or endorsement to appear in the upper portion. The portion bearing the destination address must be at least 1-½ inches high. (source)