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As we said in our earlier introduction, Tracking, a cookie is a bit of data from a web server. (Think of “fortune cookies” you might get after a Chinese meal, with little bits of wisdom inside each one.) Each web browser keeps its own set of cookies. So, if you use several computers — or several different web browsers on the same computer — each of those browsers has a different set of cookies in its “cookie jar” (actually, in the computer’s memory and/or disk).

So, for example, if you set your Google preferences on a particular browser, Google’s web server can set a cookie in that browser to maintain your preferences on that browser. But if you go to another computer, those preferences you just set on the previous computer won’t be set here because Google’s server can’t know that it’s you on that other computer. (Google has no idea where you are in a room.)

It’s possible for a web server to associate cookies with other information you enter. It won’t always do that, but it can — and often does. For instance, if you have an account and you sign in, then the web server will know who you are and that you’re using this browser. Then the web server may set cookies on that browser to “remember” that you’re using it and keep track of what you’re doing. A company’s privacy policy may explain what it stores in any cookies it sets.

Remember that, unless you have a Google Account and you sign in, Google can’t track you as a person. It can only track what’s happened on the particular browser you’re using at the moment. (This is true of other web servers, too: not just Google’s.)

You can remove the cookies from your browser by using cookie management programs or by using controls built into your browser itself. You can also prevent cookies from being set in the first place. Doing so can help to preserve your privacy, but you can also lose the advantages of cookies — such as being able to set preferences.

How Long Do Cookies Last?

Each cookie has a name and an expiration date. When a web server sends a cookie, it asks your browser to keep that particular cookie until a certain date and time. These dates can be:

  • Some date in the future. This might be a few minutes or a few hours from now (to track something like your shopping cart in an online store). Or the cookie might expire many years in the future — which means the server wants to keep track of your browser for a long time.
  • When you close your browser. This is called a session cookie. The next time you start your browser, the session cookies from the previous session will have vanished.
  • Some date in the past. This is how the server asks a browser to remove a previously-stored cookie.

As we’ll see in a moment, Google uses a mixture of session cookies and longer-term cookies.

Most web browsers let you prevent a web server from setting cookies. Add-on software can also control cookies. The most sophisticated browsers, such as Firefox, give you a lot of control over cookies.

Your browser probably has a way to remove some or all stored cookies. Doing that will stop most (but not all) tracking that a web server can do. But, of course, you’ll lose the benefits of permanent cookies. For instance, if you have a Google Account, you’ll probably have to sign in again before you use a personalized Google service like Gmail.

If you’re concerned about privacy but also want the advantages of cookies, some browsers have a good compromise: treating some or all cookies as session cookies. That is, if a server asks to store a cookie until next year, your browser can store it as a session cookie instead.

That’s enough, we hope, to give you an idea of what Google is doing “behind the scenes” in your web browser and on their servers. It’s far from everything there is to know, though! If you’d like to know more, please check the website’s privacy policy and some good references about Web security.

Cookie Examples

You can configure the Firefox browser to let you control each cookie and to see details about each of the stored cookies. Let’s use it to show a few examples of Google’s use of cookies.

Note: This section is for people who are interested in more technical details of setting cookies. If you aren’t, please skip ahead to the next chapter.

We’ll start by opening Firefox to a blank page and entering www.google.com as the URL. We’ve configured Firefox to ask before setting each cookie, and we’ve also just used its “Clear privacy data” command to erase all old cookies. As soon as we go to www.google.com, the server asks to set a cookie:

Firefox browser asking if www.google.com can set a cookie

Notice that the server wants the cookie to expire in the year 2038 and that the cookie’s name is PREF. (This may be where the server “remembers” our Google preferences.) We click the Allow for Session button, which tells Firefox to erase the cookie when we quit the browser. We could also have denied the cookie, though, to see what might happen next. It’s likely that Google will work fine with almost all cookies denied — except the cookie(s) that keep your Google Account settings.

Later, after doing some searches, we decide to sign in. Clicking the Sign in button brings up another Confirm setting cookie dialog. This time, the server wants to modify a cookie that it set earlier named GoogleAccountsLocale_session. The cookie will expire at the end of the browser session. In this case, we agree. (We could also have chosen “Use my choice for all cookies from this site” if we didn’t want to answer any more questions about www.google.com.)

Firefox browser asking if www.google.com can modify a cookie

After more searches, we open the Firefox Options dialog to look at the stored cookies. (That’s the little right-hand window in the next screen shot.) Google has set several cookies by now: five for www.google.com, one for groups.google.com, and at least one more for images.google.com. Clicking on one of the cookies shows that it’s the PREF cookie set two screen shots previous. You generally won’t need to get to this level of detail — but it is possible to, say, remove the stored cookies from a server so that server can’t “remember” you.

Firefox browser showing cookie settings and stored cookies

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This page was last modified on: Tuesday May 1, 2007

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By Nancy Blachman and Jerry Peek who aren't Google employees. For permission to copy & create derivative works, visit Google Guide's Creative Commons License webpage.