While Google has always relied on the computer version of the web pages, it is now the mobile version that will be the reference. The algorithm of Google will be based, soon, on the mobile version, so read this file to understand the SEO impact!
Mobile, Google's priority
Why does Google go to the Mobile First Index?
As Google explained in its official announcement on November, 4th 2016, the change was necessary because of the following paradox:
While at the global level most of the queries are made on mobile, Google continued to rely on the computer version to evaluate the pages. Then, if the mobile version was available, and if the user was on mobile to perform his research, then the mobile page was proposed. There were some adaptations, such as a "bonus" for mobile compatibility.
This change makes Google's analysis more consistent: if most users use Google on mobile, and offer them mobile pages as results, best use the same pages to judge their relevance. With adaptations for the research performed on computers and tablets.
That's why Google explains that its main index is that constituted by its mobile robot, the secondary index being recovered by its robot desktop. Attention, this doesn't mean that there are concrete 2 indexes! It should be understood that for Google, its index is primarily mobile, and secondary desktop and tablet.
We are still many to say "the standard version and the mobile version", but it is a mistake! The evolution of uses is such that one must really think mobile first: the mobile IS the standard version, the rest is not standard...
Precision: throughout this article, as in Google's communication, unless stated otherwise "mobile" means "smartphone". This doesn't include tablets, which are closer to computers.
Is the Google Mobile Index already in place?
No: even if the announcement is official since early November 2016, it is still in test mode. As the impact can be major (for those who have a very light mobile version for example), Google warns long in advance.
Gary Illyes, from Google, said on November 5th that it would probably not be before 4 months.