In the Returned Letter Offices of Great Britain to 1912 and Beyond (referred to as RLOGB in text), published in 2017, we described the development and roles of Returned Letter Offices (1). We also provided lists of relevant stationery items known to us at that time. These included wrappers and envelopes for undeliverable mail, detained for postage notices, form letters and labels.

The Postmaster General approved the replacement of wrappers by envelopes for returning undeliverable mail on 19 January 1852. Envelopes from Period I and early Period II show features seen on the wrappers which preceded them. These include an explanation, caution about sending valuables and an imitation seal. From then there was a gradual change in the type of envelope seen with different sizes, different purposes, different designations and different coloured paper all being seen. In addition, on several occasions standardisations of envelopes took place. These factors have led to a complex family of envelopes developing over more than 150 years. Major changes in design of or printing on envelopes are listed as subtypes. Minor changes have been excluded so as not to over complicate the Tables many of which, for example those of the PR types, are already rather long.

We have previously illustrated the reason for different designations, the range of numbers printed on envelopes (purpose and size) and devised a description-based classification (RLOGB page 185). The complete classification is shown here for the first time. It divides envelopes into seven Periods from their introduction in 1852 to 2020, near to the end of the reign of Queen Elizabeth II.

In RLOGB our description of envelopes ended in about 1920, midway through Period V. The aim of this publication is to update and complete the classification of envelopes used for returning undeliverable items. The text is divided into two parts. Part 1 describes an overview of all seven Periods of the classification. New information for and corrections to RLOGB are also included. In Part 2 envelopes of Period V are completed and those of Periods VI and VII are described. The same style has been used as in the original book.