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Understanding Envelope Window Positioning and Interior Opaques

Large proportions of envelopes are manufactured with windows, and the specifications for them can be confusing. Measurements are always taken with the flaps at the top.


Measuring a Landscape Window Envelope

Measure HEIGHT first then WIDTH second (AxB)


Measuring a Portrait Window Envelope

Measure distance in from left hand side (C) and then up from the base (D) ensuring the flap is at the top.

Types of envelope Window Film

Standard Window Film

Standard window film is around 22 micron and is approved as suitable for Royal Mail OCR reading. 


Manufactured from Cornstarch - Cornstarch window film is used throughout our Purely Environmental Evolve Recycled Range to ensure it is both recycled and recyclable.

Bespoke Envelope Windows

Bespoke envelopes can be produced with any shape window, including circular. Some promotional mailshots can have multi-windows or windows in the reverse of the envelope. Visit our customisation page to learn more.

Laser Film

Window envelopes from our Premium Business, Premium Office, Premium Pure and Premium Postfast ranges are produced with a 50 micron window film making them guaranteed to withstand the heat of both colour and mono laser printers and digital machines.

Reverse Windows

We also stock a collection of envelopes under our Purely Everyday range with windows on the reverse side of the envelope, these are especially designed to leave the face area of the envelope completely free for full face printing.

Close-up image of envelope opaque


Printing the inside of the envelope or interior opaque, as it is known, is designed to enhance the security of the contents. This is almost exclusively restricted to white envelopes.

An interior opaque can also be used to reveal a message through the window once the contents have been removed. We use this in both our Purely Environmental Recycled and our Purely Environmental FSC ranges to reinforce their environmental credentials.

Most interior opaques are printed in shades of blue or grey during manufacture. However, on special bespoke orders if the quantity is large enough, customers can have their own design opaque or specified Pantone colour. Some choose to incorporate names, logos or messages in the throat of the envelope which is referred to as a lozenge.

Our blueprint opaque is the ‘mountain peak’ design, printed in Pantone 292 and consistent across the complete range. This opaque has proved very popular, so much so, some have tried to imitate it. We also offer a few select lines with a grey opaque.

We have introduced the brand logos in the throat of our Premium and Creative Colour envelopes to raise brand awareness.