It used to be that everyone wanted a bigger computer monitor than what they had. Bigger was better, so web developers designed sites with a fixed width that would fit on all monitors and then just extend out on the “wings” to fill up whatever extra space there was.

Then Apple came along and invented the iPhone and that made things complicated. Now computer screens were really small. So the concept of “mobile responsiveness” was born. Websites weren’t just static anymore, now they had to scale to fit the tiny screen of a smartphone.

But smartphones didn’t just change the way that developers sized websites, they also changed the way that we as users interacted with websites. Or perhaps more accurately, they allowed us to interact with sites. Suddenly we weren’t just dragging a scrollbar and using a mouse to click text. Now using websites was tactile—we could swipe and tap instead of scroll and click.

And because smartphone screens were so much smaller than their desktop relatives, we found ourselves doing a lot more swiping when using the internet than we were used to. A website page that is 1024 pixels wide and 768 pixels tall on a desktop might be 360 pixels wide and thousands of pixels tall on a smartphone. An iPhone 5s screen is only 568 pixels tall, so you can see how you have to scroll to see most of the site, even when it’s responsive.

So developers started thinking, “what if websites did something cool while you scrolled through the content on a smartphone?” Enter the parallax website: a site where layers of text and images move at different speeds as you “swipe” (or scroll) down the page. Think of driving a car and looking out across a field. The fence in the foreground is whizzing by, while the barn in the distance seems to move much more slowly. This is parallax in real life.

Typically, parallax websites are a single page instead of ten or twenty pages. This lets you scroll through the whole site in one place and get the full “parallaxing” effect. Because there’s less real-estate for content on one giant page, parallax sites are designed to make a few strong points and invite users to call or find out more about the business. This makes them useful for some businesses but not suited for all applications.

They’re great for calling attention to one really important message but not so good for a site that relies heavily on, say, large databases of products and information. Think of them as slick, quick advertisements, rather than long-winded written explanations. Parallax sites move beyond a “brochure” mentality, where you have to click all over the site looking for pages and reading lots of words.

Someone on the go using a smartphone doesn’t have the time (or the vision for that matter) to do a lot of reading and clicking. They want to hit the high points about your business and get the information that they need quickly.

So when is a parallax site the right decision?

1. When you want to shift more toward advertising on your website
2. If you want a new cutting edge look to what you’re advertising
3. If you can use succinct, compelling messages to get your ideas across
4. When you want to appeal to smartphone users
5. If you only have one or two very strong, clear points to make

A parallax site is probably not the right kind of website to be using if:

1. You need to give your user a very tradition user experience
2. Your users tend to be older or less interested in new technology
3. You have a large and varied amount of data, pictures and text that you need to put on your site