What shape must a postcard be to meet postal requirements?

A postcard must be rectangular to meet postal requirements due to standardization and machinery used by postal services worldwide. The rectangular shape allows for efficient handling and processing, ensuring that postcards can be easily sorted and delivered without complications.

The dimensions of a rectangular postcard are also specified by postal regulations, which typically require a minimum and maximum size for proper mail processing. This shape is optimal for addressing, as the space can accommodate the necessary information such as the recipient's address, stamp, and any messaging succinctly while maintaining clarity and functionality for mail delivery.

Shapes like square, circular, or triangular may deviate from the established norms and could lead to difficulties in processing and automated sorting techniques utilized by postal services. Therefore, adhering to the rectangular shape ensures compliance with postal regulations and leads to a smoother mailing experience.