What shape must a postcard be to be mailed?

The correct answer is that a postcard must be rectangular in shape to be mailed. This requirement is based on postal regulations that define the accepted dimensions and shapes for postcards to ensure that they can be processed efficiently through sorting and delivery systems. Rectangular postcards adhere to standard size guidelines, making them easy to handle and sort within postal facilities.

Square postcards may be appealing from a design perspective but can create complications in automatic sorting machines, which are typically optimized for rectangular items. As for round and oval shapes, these are generally not accepted for mailing as they can pose challenges in processing and may not fit into standard postal equipment. Thus, rectangular postcards are the only shape that meets the necessary criteria for mailing.