This document covers physical properties and manufacturing requirements for envelopes having an address window on the flap side. It covers the main design features of the reverse envelope@ notably of the flap and address window@ and the materials used for the manufacturing thereof. It applies to reverse envelopes with advertising or communication printed on the plain side@ eventually on its entire surface. This document covers empty envelopes@ but also finished mailpieces that have been properly inserted@ addressed and franked (reversed mailpieces) and are submitted to Postal Operators. In particular@ reverse mailpieces will be compliant with relevant Postal standards applicable in the member states. By extension@ these requirements also apply to non-window envelopes used for reverse mailpieces and having the address printed on the flap side. This document does not apply to: ?? envelopes with a large window on the plain side (opposite to the flap) as these are already common and widely accepted; ?? paper requirements to ensure print quality (except for the postage mark and address) and notably colour rendering.