...that was recently confirmed by an official Google representative in WebmasterWorld, is in the favor of one domain per Ad Group and will disallow multiple display domains per ad group.

In an effort to provide more relevant results and a high quality experience for our users, we've made the decision to disallow multiple display URL domains within a single ad group. Going forward, all display URLs within an ad group must have the same top-level domain. Please note that this amendment to our policy applies to all advertisers, regardless of previous exceptions or acceptability of any ad groups.

See the AdWords blog post for more official words from their end.

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