Perhaps the most familiar of these textile panels is the dossal, sometime called a “dorsal” which refers to the back curtain panel behind the altar where generally a reredos would be found. Most likely the original word was dorsal, and dossal is a corruption of the word. A dossal may be a flat panel with a central large motif or it may be a gathered width of textile. Both types are suspended by a strong iron rod which is held in place by rod holders secured in the stone or wood. It seems likely that dossals had their origin in order to give dignity and beauty behind altars which had no beautiful carved or painted altarpiece or reredos. The addition of riddels, or side curtains might be a further enhancement for the altar or perhaps a practical draft blocker in chilly, drafty old stone cathedrals. In the photo to the left, four elaborate riddel posts are visible from which horizontal support rods are suspended for hanging the side curtains. The combination of dossal and riddels is not uncommon in the United Kingdom. A favorite textile pattern for these curtains is a small all-over floral or a tapestry of multicolor floral which will harmonize with all frontal colors. A plain unbleached linen is shown in the photo to the left as the curtain choice because the altar is dressed in Lenten array.
Below is pattern Braganza - a popular choice for a dossal along with Portuguese, Verona, Coronation
It is more rare to find a tester which overhangs the altar as a sort of canopy and is usually attached to the top of the dossal or is in fact simply all of a piece draped over a tester frame (see photo below).
The photo below shows a half-dossal and riddel arangement with a veiled hanging pyx suspended above the altar . The frontal with attached superfrontlet is in the familiar Portuguese tapestry pattern shown here with an attractive diced fringe. Note the riddel posts.
From A Dictionary of Architecture and Landscape by James Stevens Curl
riddel(I) or riddle. In a church, the curtains suspended around an altar, sometimes from rods fixed into the wall behind, but more often from some means of hanging spanning between riddel-posts: there were normally four of the last, polygonal on plan, coloured and gilded, and crowned by angels, often supporting candelabra. Arrangements of riddels behind and around altars seem to have been not uncommon in England towards the end of the Gothic period, in the decades immediately before the iconoclasm of C16, and were revived in the early C20 during the late flowering of the Gothic Revival, notably by Comper and Temple Moore.
Comper (1893, 1897, 1933, 1950);
Dearmer (1911, 1931);