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Thomas great info above, quick follow up question for you..... I have always wonder why is Google using the "640px" in the   ? many people have been asking the question lately if 640px is an old example or is it the required size? (phones are larger nowaway) the website I manage are non-responsive ocated in a /mobile/ folder such as: http://www.example.com/mobile/page1  and our mobile size cut off is actually 1023px. .... should we be using 640 or 1023px in the rel="alternate"  tag? thank you!