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If you see symbols like these scrawled in chalk on buildings near where you work, it's not part of a noughts and crosses craze. It's all to do with finding where you can take your laptop and get online - for free. Trying to find ways of communicating without paying has a long and - some would say - honourable tradition.

In years well before the internet, all sorts of tricks were used to explore phone networks and to con free calls out of phone companies.

The process of trying to find phone lines being used by computers that were ripe for hacking was dubbed "wardialling".

When many companies set up their own wireless networks within their buildings - meaning their employees could use their laptops without having to use cables - it opened all sorts of possibilities.

People on the look-out for free access would drive, or walk, past offices in the hope of finding areas where the wireless access "spilled" into the street. That was termed "wardriving" and "warwalking".

Now, these strange symbols are the newest twist on the process, as they show passers-by where someone has discovered one of these areas where they can get online.

It is called "warchalking"; since the word was coined just one week ago, the concept has gone round the world, with symbols appearing in several cities.

Before warchalking, the only way to find out about these wireless access points, or nodes, was to find out about them on the web or spend a long time carrying out your own survey.

But warchalks provide a shorthand to help people find and use the wireless network nodes, which are springing up in cities and towns all over the world. They work just as well for the co-operative "open" networks, where anyone is invited to take part, as they do for corporate ones.

Each warchalk is designed to reveal key information about the node to help people use it.

There are different signs for open, closed and encrypted nodes.

The information supplied in warchalks for open networks should give people enough to let them tweak their wireless network settings and use the wireless access.

The symbol for an open node is two back-to-back half-circles annotated with the available bandwidth and an ID to help people join it.

The idea came from net design expert Matt Jones, the original designer of BBC News Online, who first wrote about it on his weblog Black Belt Jones on 24 June.

It swiftly caught on, and from receiving about 200 visitors a day to his website, the number ballooned to almost 10,000.

The explosion came after his site was mentioned on the popular Slashdot and Metafilter websites, and now Jones has received inquiries from Barcelona, Seattle and San Francisco. The first "wild" warchalks are starting to show up.

"It's a bit overwhelming to be honest," he said.

"As someone who works on design and the internet and trying to build communities," he said, "having that happen in one day is pretty astonishing."

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The original idea emerged during a lunch Jones had with friends who are also part of the wireless, web and weblogging world.

Jones mentioned to them that he had seen some students who had chalked an office plan on a London street and were sitting on office chairs using laptops and a wireless network in a parody of a conventional workplace.

One of the people sharing lunch mentioned the symbols hobos used to let their fellow travellers know where to find charity, a warm bed or trouble.

"The idea just bubbled up from there," said Jones. "It's not really got a single author."

The reason for using chalk rather than paint is to ensure that the signs will have to be rewritten regularly and the information in them maintained.

"It's kind of interesting that people will have to go around and keep renewing it," said Jones.

The warchalks are intended to let people know about the open nodes that many people are happy for others to share.

Jones said he did not think there was much danger that it would be taken over by anyone malicious to post notice of corporate networks that are not doing enough to protect themselves.

"If someone is chalking it up they are doing something quite beneficial to the network operators by saying I have spotted this. Then they can decide to secure it or instigate a free wireless type scenario."

At a time when swathes of invasive laws and other measures are bing ushered in under the guise of anti-terrorism, this is a wonderful development. State surveillance and monitoring generally brings more harm - repression, paranoia, misuse of personal information etc.. - than benefit. Real security comes only from people and co-operation: the spontaneous peaceful anarchy of warchalking is a cause for hope.
David Pollard, UK