If I write a new app in W10M will it work on 8.1 devices?

[Windows Central Question]

I have to write a mobile app for W10M, but am concerned it will not work for users with earlier versions. Anyone know?

 

[Tumultus]

It probably won't because you'll be using built-in libraries that aren't available on WP8.1. If you would want to target both, you would have to target the Windows Phone 8.x platform and mark it compatible to W10M when you publish it. Downside: You won't be having a universal app that also works on desktops.

Why don't you just rethink your strategy before you get started? Would you need support for WP8.x devices or would it be better to target W10 and use the new libraries with much more features? Those are the questions you should ask yourself.