thanks! I don't know much about the advertising industry, I just thought about the following question: if some site is hosting a pop-up advertisement, then the javascript on that advertisement can get the referrer header by means of its own javascript code. Now my question is: do the different advertising networks share that data between one another? I guess that it might be a big business, if yahoo ads would trade the obtained glcid parameter values with google, or vise versa.