If you've yet to build a great mobile version of your website, here's a big incentive to do it: Google is rolling out mobile-first indexing, so it will use the mobile version of a site for indexing and ranking rather than the desktop version. That's according the company's official Google Webmaster Central Blog.

This is a bigger deal than you might realize. It means that if you've got a desktop version of a website and a mobile version of your website, and the mobile version is a slapdash affair, your Google search rankings will likely suffer. And if you have no mobile version, even though your desktop version will be indexed, your mobile search results will suffer. And both those things mean fewer visitors to your site.

Google didn't take this step lightly. It's been testing doing it for a year and a half. And it's doing it because mobile searches on Google outnumber desktop searches. The blog explains that until this move, Google's "crawling, indexing, and ranking systems have typically used the desktop version of a page's content, which may cause issues for mobile searchers when that version is vastly different from the mobile version. Mobile-first indexing means that we'll use the mobile version of the page for indexing and ranking, to better help our - primarily mobile - users find what they're looking for."

Google recommends that webmasters check out Google's developer documentation about how it determines the mobile content of a site. The documentation, Google says, "covers how sites using responsive web design or dynamic serving are generally set for mobile-first indexing. For sites that have AMP and non-AMP pages, Google will prefer to index the mobile version of the non-AMP page."

Google says that if a site isn't mobile-friendly or doesn't have a specific mobile version, the desktop version will be indexed. But, the company notes, "we do evaluate all content in our index -- whether it is desktop or mobile -- to determine how mobile-friendly it is. Since 2015, this measure can help mobile-friendly content perform better for those who are searching on mobile."

Mobile-first indexing is just being rolled out now. Google is notifying sites when they'll be mobile-first indexed via Search Console, so check out that tool regularly. And if you don't have a mobile-friendly or mobile version of your site - or if the one you have isn't up to snuff - it's time to get working on one.