The width of the black border on a mourning cover indicates how close the relationship was to the deceased and the depth of grief. Mourning covers were used throughout the world. They are still used, although rarely, today.
“Mourning covers” refers to black‑bordered postal envelopes used historically to send death‑related correspondence (see Fig. 1).
The term traditionally refers to black‑edged postal covers used in the 19th and early 20th centuries to signal bereavement. These envelopes were sent to notify recipients of a death or to convey condolences.
The border may start thick and thin over time, or the thickness may depend on the level of bereavement (see Fig. 2), or both. For example, the death of a child may call for a thicker border than the death of a cousin. Social norms expected that the mourning period was to be displayed in both public and private. After the death of a reigning monarch, a mourning border may be placed on public notices, newspapers, and other government stationery.
Mourning stationery is a letter, envelope, or calling card with a black border (see Fig. 3 and 4), used to signify that a person is experiencing mourning. They were first used in the 17th century in Europe and were most popular during the Victorian era, during which it was also used in the United States and West Africa.
The convention was that children did not use mourning stationery. If seals are used with mourning stationery, they are conventionally black. Black borders may also be used on letters announcing an execution.
Source: wikimedia.org