The Unlisted Securities Market (USM), which ran from 1980 to 1996, was a stock exchange set up by the London Stock Exchange. It served a market for shares of companies too small to qualify for a full listing; or which did not have the full three year trading history required by the main market; or which wished to float less than 25 percent of their share capital (required by the main market while the USM allowed as little as 10 percent free float).

The Alternative Investment Market (AIM) was set up in June 1995. The USM no longer accepted new introductions, and companies which were already quoted on the USM had twelve months to decide either to move the quotation to AIM, or to delist.

The American equivalent is called Over the Counter (OTC) trading.

[edit] References

"Her Majesty's Revenue & Customs". Capital Gains Manual Definitions: stock exchange: quoted/unquoted: unlisted securities market (CG50256). http://www.hmrc.gov.uk/manuals/cg3manual/CG50256.htm. 

Buckland, Roger; Davis, Edward W (12 October 1989). The Unlisted Securities Market. USA: Oxford University Press. p. 192. ISBN 978-0198285267.