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The Stock Exchange Automated Quotation system (or SEAQ) is a system for trading mid-cap London Stock Exchange (LSE) stocks. Stocks need to have at least two market-makers to be eligible for trading via SEAQ.

In the LSE, only thinly traded stocks (those that are not part of the FTSE-100) are traded on the SEAQ. It is a quote-driven market made by specialized and competing dealers, also known as market-makers.

The system contains no public limit orderbook and as such, the system is not MiFID compliant. For this reason, the LSE developed the SETSqx system which allows members of the public to display limit orders, although these orders are traded through so that typically incoming market priced orders will trade with the 'Market Makers Quotes' and not at better prices, which may be available inside the market makers Bid/Ask spreads.

SEAQ effectively gives a monopoly over all of the incoming orderflow to the market makers since members of the public cannot submit limit orders which can match with each other (because of the lack of a proper orderbook). Hence, all deals on SEAQ have to be done at the market makers prices.

Bid/Ask spreads and hence trading costs on SEAQ are typically very high due to its anti-competitive market structure.