Just over a year ago, Google announced a major change to the way its search index will rank sites: it will go mobile-first. As the company explained at the time, Google’s algorithms will eventually primarily use the mobile version of a website’s content to rank its pages, to understand structured data, and to show snippets from the site in its search results. This week, Google said it has begun to make this shift – and has already transitioned a handful of sites to the mobile-first index, which are now being monitored by the search team.

The goal with the change to a mobile-first index has to do with how people access the web today – that is, they do so on their mobile devices, like their smartphone. In fact, the company said last November that “most people” today are searching Google from mobile.

But Google’s ranking systems for webpages are a holdover from the days of the desktop web. That is, they still typically look at the desktop version of a page’s content in order to determine how it should be ranked and its relevance to the user.

In many cases, that’s fine, as the desktop and mobile site are the same. Sometimes, however, the mobile page has less content than the desktop page, which can cause the system to be less accurate. For web searchers, that means they may be pointed to a page that doesn’t have the information they were looking for, and then blame Google –  not the website for having a poor mobile site – as a result.

Google said last year it understood a shift to a mobile-first index was important and needed to be taken seriously, which is why it would experiment on a small scale before ramping up to extend the index to cover more sites.

The announcement this week that Google has now shifted a series of websites to the mobile-first index was noted at the bottom of a “how to” guide for web publishers, which explained how to get their sites ready for mobile-first indexing.

Google noted that a mobile-first index means that webmasters will see “significantly increased” crawling by its Smartphone Googlebot. It also means the snippets in the results and the content on the Google cache pages, will be from the mobile version of the pages, when live.

Pages that already take advantage of responsive web design and implement dynamic serving don’t usually have to do anything to be ready, Google added.

At the end of the guide, the company noted that it has started mobile-first indexing for a handful of sites, which are being monitored by the Google Search team.

When asked, Google declined to comment on the change of some sites to a mobile-first index, but the company has been working on the project since its announcement last year, speaking about it at events, and working with webmasters. It had not before given an indication of how far the mobile-first index had rolled out.

While this does indicate the index is closer to launch, the company has estimated that it likely won’t go live until 2018 at the earliest.

Google had previously said that when it was ready to migrate sites to a mobile-first index, it would begin with those that were ready for it. The company didn’t say that had taken place, instead saying that “it would have more to announce around that later on.”