For mail sorting purposes, an envelope is divided into four zones as described below.

Keep clear zone

The keep clear zone is a 15mm-high strip along the bottom edge of the envelope and extending up the right edge to the postage zone at a width of 15mm. This zone is reserved for barcode printing and sorting.

Postage zone

The postage zone is located in top right-hand corner of the envelope. It is 90mm wide and 40mm high. This zone is reserved for stamps, postmarks and postage meter imprints.

Service zone

The service zone occupies the top left-hand corner of the envelope, adjacent to the postage zone at a height of 40mm. It also includes a 15mm-wide vertical strip along the left edge of the envelope, above the keep clear zone. The service zone is reserved for service indicators such as advertising or a return address.

Address zone

The address zone is an area of variable height and width located roughly in the centre of the envelope. It is enclosed by the keep clear zone below and to the right, the service and postage zones above and the service zone to the left.