Google made an important announcement on their webmaster blog on Monday.

Today we’re announcing that after a year and a half of careful experimentation and testing, we’ve started migrating sites that follow the best practices for mobile-first indexing.

To recap, our 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.

We continue to have one single index that we use for serving search results. We do not have a “mobile-first index” that’s separate from our main index. Historically, the desktop version was indexed, but increasingly, we will be using the mobile versions of content.

You can read the whole thing here if you’re so inclined: Rolling out mobile-first indexing

Or, you can continue reading for a handy Google-to-English translation.

Still here? Cool. Here goes.

As you probably know, Google uses tools – commonly referred to as “spiders” or “crawlers” – to record and catalog the information on web sites. They then use that catalog to match the questions that people type into search forms – search queries – with relevant information, or search results.

Still with me? Good.

Now… many web sites look and behave a bit differently when viewed on a phone or a tablet, as opposed to a desktop computer. The desktop version was always considered the default, the authoritative version; the tablet and mobile views were secondary.

Until now.

As anyone that’s been paying attention already knows, more and more web usage is shifting to mobile devices. Google recognizes this, and they’re going to start adapting their search results accordingly.

And this is where things get tricky for the average web publisher.

Does your site work well on mobile devices?

If your site is more than a couple of years old, odds are it doesn’t. You can (obviously) check it out on your phone to see… you can even “fake” the mobile experience by making your browser window very narrow. (That method doesn’t always work, but that’s a different post.) But the best way to check is probably to use a tool like Google’s Mobile Friendly Test.

Plug in your URL, and Google will give you an idea of how well your site works on mobile platforms, a rendering of what your home page looks like on a typical phone screen, and even some extra-geeky technical details… errors, objects that failed to load, good stuff like that.

Look at those results… that’s what around half your site’s visitors are seeing.

If you’re not happy with that situation, give us a shout.