Apparently, having looked into it, it's Google showing an empty space (or if the Adsense account holder chooses, a coloured square or a static image) if there are no relevant ads to display.
I don't know what criteria they use to determine what content to use to display the advert, I would presume page content but there doesn't appear to be any concrete information.
And I swear there used to be an option to "Show Public Service Ads" when there were no relevant Ads, but that option appears to have disappeared from the Adsense control panel.
Bleh.
So, if someone (hypothetically speaking) were to start a discussion about, say, handbags and handbag related products, we might get some adverts appearing from Google?
Bags, Thomas Tank Engine paraphernalia, Password recovery tools. Anything
Indeed. There have to be limits. We have a classy spamming portfolio and we don't want to drag it down-market.
I got some ads today! Shock.
I AM BEING ADVERTISED AT TOO.
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