Windows 8.1 and Windows Phone 8.1 have different display sizes and behaviours and so you need to create two user interfaces. These are managed as two separate projects in the universal solution. However, if you are clever with your design you can put all of your business logic in the shared code and then just project the appropriate view onto the target devices.

On the right you can see how this all fits together. I'm making a QuickPrint application that lets you send things to a little Bluetooth printer that I'm putting together. The two versions of the application each have their own MainPage and then there is a project for the shared code. In this case I'm sharing my Bluetooth manager code.

I was very surprised (and pleased) to find that the Bluetooth manager worked on both platforms in exactly the same way. It does some fairly low level Bluetoothy things, and they just worked.

As a Windows Phone developer moving over to the Windows 8 platform there are some differences in the way that the XAML works, but nothing that can't be solved by using Intellisense and a few searches. And at the end of the process I had something that I can think about deploying in both marketplaces.

And of course this means it is now even easier to become a phone developer if you have already written some Windows 8 stuff, which is lovely.