In 1971, the 13-volume OED1 (1933) was reprinted as a two-volume Compact Edition, by photographically reducing each page to one-half its linear dimensions; each compact edition page held four OED1 pages in a four-up ("4-up") format.
The two volume letters were A and P; the first supplement was at the second volume's end.
The Compact Edition included, in a small slip-case drawer, a magnifying glass to help in reading reduced type.
Many copies were inexpensively distributed through book clubs.
In 1987, the second supplement was published as a third volume to the Compact Edition.

In 1991, for the 20-volume OED2 (1989), the compact edition format was re-sized to one-third of original linear dimensions, a nine-up ("9-up") format requiring greater magnification, but allowing publication of a single-volume dictionary.
It was accompanied by a magnifying glass as before and A User's Guide to the "Oxford English Dictionary", by Donna Lee Berg.
After these volumes were published, though, book club offers commonly continued to sell the two-volume 1971 Compact Edition.

The Compact Oxford English Dictionary (second edition, 1991, ISBN 978-0-19-861258-2): Includes definitions of 500,000 words, 290,000 main entries, 137,000 pronunciations, 249,300 etymologies, 577,000 cross-references, and over 2,412,000 illustrative quotations, a magnifying glass.

?
th impression (1991-12-05)
In 1971, the 13-volume OED1 (1933) was reprinted as a two-volume Compact Edition, by photographically reducing each page to one-half its linear dimensions; each compact edition page held four OED1 pages in a four-up ("4-up") format. The two volume letters were A and P; the first supplement was at the second volume's end. The Compact Edition included, in a small slip-case drawer, a magnifying glass to help in reading reduced type. Many copies were inexpensively distributed through book clubs. In 1987, the second supplement was published as a third volume to the Compact Edition.
In 1991, for the 20-volume OED2 (1989), the compact edition format was re-sized to one-third of original linear dimensions, a nine-up ("9-up") format requiring greater magnification, but allowing publication of a single-volume dictionary. It was accompanied by a magnifying glass as before and A User's Guide to the "Oxford English Dictionary", by Donna Lee Berg. After these volumes were published, though, book club offers commonly continued to sell the two-volume 1971 Compact Edition.
The Compact Oxford English Dictionary (second edition, 1991, ISBN 978-0-19-861258-2): Includes definitions of 500,000 words, 290,000 main entries, 137,000 pronunciations, 249,300 etymologies, 577,000 cross-references, and over 2,412,000 illustrative quotations, a magnifying glass.
?th impression (1991-12-05)
Oxford English Dictionary