The Stock Exchange of Hong Kong (SEHK) is the largest and oldest stock exchange in the country and is a major component of HKEx. Founded in 1891 as the Association of Stockbrokers in Hong Kong, it officially became the Stock Exchange of Hong Kong in 1914. SEHK is one of the most actively traded exchanges in the world and is home to some of the most reputable and well-known corporations in Asia. 'H-shares,' stocks listed on the Shanghai and Shenzhen stock exchanges but owned by companies in Hong Kong, are also traded on the SEHK.

The Hong Kong Futures Exchange (HKFE), a wholly owned subsidiary of HKEx, was founded in 1976 and is responsible for trading of futures and options contracts. The exchange operates derivatives markets for commodities, equities, interest rates, and foreign exchange. As an established financial institution, the HKFE is highly respected and has been rated as one of the most reliable exchanges in Asia.

HKEx also owns the London Metal Exchange (LME). This was previously a London-based company established in 1877 that dealt in trading of non-ferrous metals. The LME operates as an international marketplace for the buying and selling of a wide range of primary and semi-finished metals and is known for being the world's leading metals trading platform. In 2012, HKEx acquired the LME in a £1.39 billion transaction and has since increased its exposure to the Chinese market by trading metals such as copper, aluminum, lead, zinc, tin, and nickel.

The four clearing houses owned by HKEx are subsidiaries of the company and are responsible for clearing derivatives and trading activities associated with the HKEx markets. The four clearing houses are Hong Kong Securities Clearing Company, Hong Kong Futures Exchange Clearing Corporation, Shanghai-Hong Kong Stock Connect Clearing House, and London Metal Exchange Clearing House. The clearing houses provide a secured and efficient process for the swift settlement of transactions conducted within the various exchanges operated by HKEx.

In conclusion, Hong Kong Exchanges and Clearing Limited (HKEx) is a publicly-traded holding company that operates the Stock Exchange of Hong Kong, the Hong Kong Futures Exchange, the London Metal Exchange, and four clearing houses. Companies wishing to be listed on the main board must pass one of three tests, and the services of HKEx include listing, trading, clearing, market data, reference prices, and connectivity. Through its various exchanges and subsidiaries, HKEx plays an integral role in the region's financial markets by providing a secure and efficient trading platform for investors, companies, and metal operations around the world.