URLs of all links the page contains. As soon as the user follows one of these links, the corresponding URL is requested to the server. This way, the download of this page is linked with the previous one. For example, when a web page containing the following is requested: <a href="foo.html">see my page!</a> <a href="bar.html">mine is better</a> a unique string, such as e0a72cb2a2c7 is chosen, and the page is modified as follows: <a href="foo.html?e0a72cb2a2c7">see my page!</a> <a href="bar.html?e0a72cb2a2c7">mine is better</a> The addition of the query string does not change the way the page is shown to the user. When the user follows, for example, the first link, the browser requests the page foo.html?e0a72cb2a2c7 to the server, which ignores what follows ? and sends the page foo.html as expected, adding the query string to its links as well. This way, any subsequent page request from this user will carry the same query string e0a72cb2a2c7, making it possible to establish that all these pages have been viewed by the same user. Query strings are often used in association with web beacons. The main differences between query strings used for tracking and HTTP cookies are that: Query strings form part of the URL, and are therefore included if the user saves or sends the URL to another user; cookies can be maintained across browsing sessions, but are not saved or sent with the URL. If the user arrives at the same web server by two (or more) independent paths, it will be assigned two different query strings, while the stored cookies are the same. The user can disable cookies, in which case using cookies for tracking does not work. However, using query strings for tracking should work in all situations. Different query strings passed by different visits to the page will mean that the pages are never served from the browser (or proxy, if present) cache thereby increasing the load on the web server and slowing down the user experience. Compatibility issues According to the HTTP specification: Various ad hoc limitations on request-line length are found in practice. It is RECOMMENDED that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets. If the URL is too long, the web server fails with the 414 Request-URI Too Long HTTP status code. The common workaround for these problems is to use POST instead of GET and store the parameters in