The mail or post is a system for physically transporting documents and other small packages, as well as a name for the postcards, letters, and parcels themselves. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century national postal systems have generally been established as government monopolies with a fee on the article prepaid. Proof of payment is often in the form of adhesive postage stamps, but postage meters are also used for bulk mailing. Modern private postal systems are typically distinguished from national postal agencies by the names "courier" or "delivery service".