Negative Keywords are used to suppress ad impressions for undesirable keywords.
They are extremely valuable as they ensure your ad doesn’t show for something you don’t offer, like "free".
Using them causes your CTR (clickthrough rate) to rise, giving you better relevancy and Quality Score, and the possibility to pay less for your clicks (hence buy more of them for the same budget).
You should have as many of them as possible, and if using Broad match keywords they are critically important.
There’s no limit to how many you can have in your campaigns, and I have accounts with many thousands of negatives – sometimes more than ordinary keywords.
But beware! Choose the wrong ones and they will turn off your ads when you *want* them to show…
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