THE first e-mail sent from China, on September 14th 1987, was optimistic: "Across the Great Wall we can reach every corner in the world." Few of China's 560m internet users now have such reach, however, because China tightly controls its people's use of the internet. The "Freedom on the Net 2012" report, issued by Freedom House, an American organisation that tracks global trends in political freedom, ranked China as the third most restrictive country in the world when it comes to internet access, after Iran and Cuba (though this ranking excludes those places, such as North Korea, where ordinary people are not allowed to use the internet at all). How does China censor the internet? The Chinese central government has two main ways of controlling what its citizens see on the web: the Great Firewall, as it is called by foreigners, which is a system of limiting access to foreign websites which started in the late 1990s, and the Golden Shield, a system for domestic surveillance set up in 1998 by the Ministry of Public Security. Separate government departments, along with local and provincial administrations, also have their own monitoring systems. China began by blocking a list of foreign websites, including Voice of America, human-rights organisations and some foreign newspapers. But its filters have since become more sophisticated and can now selectively block specific pages within foreign websites, rather than making the entire site inaccessible. They can also block particular terms when they are used in search queries or instant messages. Google is not blocked entirely; instead, users who search for banned keywords are blocked from Google for 90 seconds, though other websites remain available. China's many internet companies are regularly issued with lists of restricted keywords, and often censor blog posts and other content pre-emptively to avoid trouble with the authorities. In all there are thought to be around 100,000 people, employed both by the state and by private companies, policing China's internet around the clock. Since 2005 the state has also paid people, known as the "50 Cent Party", to post pro-government messages and steer online conversations away from sensitive topics.

China's criteria for censoring the internet are slightly more subtle than foreigners often assume. In essence it applies the rules that have prevailed since the Tiananmen Square crackdown of 1989: do not jeopardise social stability, do not organise and do not threaten the party. Accordingly, criticism of mid-ranking officials is tolerated, particularly if it is in keeping with the government's anti-corruption drive. But attacks on the senior leadership are swiftly removed (prompting Chinese internet users to refer to senior figures using nicknames or coded language, in an effort to stay ahead of the censors). The most brutal restrictions are applied to any post that calls for offline protests or demonstrations—even for pro-government causes. The censorship system's main goal is to prevent the internet from being used to co-ordinate or organise real-world political activity. In extreme cases, internet access may be cut off altogether, as happened for ten months in 2009, after riots in Xinjiang, a remote north-western region.

In short, China is having it both ways: it is allowing its citizens to benefit from the social and commercial aspects of the internet, while placing strict limits on its use for political activism. Other authoritarian governments consider China's approach a model to be emulated. There is no doubt that microblogs such as Sina Weibo, the Chinese equivalent of Twitter, have given the public a new voice with which to demand more accountability from officials on issues such as corruption, food safety and air pollution. But so far the government has managed to prevent the internet being used to campaign for broader political change. Indeed, by providing people with an outlet to vent their concerns and giving the illusion of public debate, the internet may even be delaying the radical changes that China needs.

• What else should The Economist explain? Send us your suggestions.