How a postmark turns postage into a one-way trip
A stamp works as evidence that postage has been paid, but only if it cannot be transferred to another envelope. At a post office, the stamp is pressed or run through a machine that applies an ink cancellation—often a set of bars or wavy lines across the stamp. The ink obscures enough of its design to make a second use conspicuous, while the cancellation also signals to postal workers that the stamp has already paid for a shipment.
This is why the mark is placed over the stamp rather than merely somewhere on the envelope. A clean, unused stamp can still look valid; a cancelled one has been visibly spent. The envelope can then move through sorting and delivery without relying on every worker to remember which stamps have already passed through the system.
A working mark can carry several clues
Many cancellations combine the defacing pattern with a post office’s name or code and a date. Those details can help establish where and when a letter entered the mail stream. They are useful operational clues, not a complete travel diary: a letter may pass through several sorting centers, and its cancellation generally identifies the processing point that applied the mark, not every place it later went.
For the sender, the practical result is simple: once the mark crosses the stamp, the postage has been used. For the postal service, one quick impression both helps prevent reuse and leaves a trace of processing—turning a small patch of ink into a control point for a journey that may span many handoffs.
