To determine the mailability or machinability of a letter-size mailpiece, the length is the dimension parallel to the delivery address as read; the height is the dimension perpendicular to the length. Because the location of the delivery address on a letter-size mailpiece determines which dimensions are the length and height, the placement of the address may make a piece nonmailable or nonmachinable.
Note: When listing dimensions, the normal practice is to list length first — e.g., a dimension listed as 4" by 6" usually indicates that the length is 4".
