This page explains in detail the "Mobile:" setting in each block's Permissions tab.

Once upon a time there were just PCs and a 12 inch screen was pretty good.   Then there were laptops, and notepads, and then mobile phones with snall screens, then with larger screens, then tablets and phablets, and ... .   Ugly truth #1 is that your site may be browsed by any number of devices and screens sizes, it is not merely mobile and non mobile.

Not all screens are the same, and attempting to show the same content to all screens makes some users of some screens frustrated. Ugly truth #2 is that it is your site's problem to deal with the unpredictable variety of screen sizes.

The way to deal with these ugly truths leads to ugly truth #3 - you have to give up the idea that you can exactly control how your site looks.  You can try to exactly position each element, decide just how much spacing you want after the sidebar,etc, but whatever you decide will most likely work only for the screen size you were using, and when viewed from a very different screen size may look quite different.  Instead you have to let the browser move things around to try to show all that needs to be shown.

Ugly truth #4 is that you simply cannot show as much on a small screen as on a larger one. Folks just do not love your site so much as to scroll endlessly reading your lovely content.

The mobile setting causes blocks to appear and disappear based on the screen width.   With it you can choose not to show a block when the screen is small, or to show alternative blocks.

The toggling is based on the following grouping of screen sizes:

• Mobile – 767 pixels or smaller
• Tablet – 768 to 921 pixels
• Laptop – 922 pixels and up

The terms "Mobile" "Tablet" and "Laptop" are just labels that match when these sizes are commonly found. The ugly truth about device variety is that there are no such simple categories.

With a mobile setting the block will come and go, dynamically, depending on screen width.   The block is actually always present, it just is shown or not by modern browsers in response to "media query" rules.

This effect is easiest to explain by example. Below are blocks that come and go. Resize this browser window to see the effect of the Mobile settings.

When "Mobile:" is set the blocks may come and go. Enable Configure Blocks on the toolbar to have a placeholder appear in the place of any suppressed block.