Universal app for iPhone SDK 4.0

I have updated my SDK to the latest version: 4.0. So now I am able to create iPhone apps and iPad apps. I want to create a universal app. How can I do this?

If we created a universal app, does this mean this app works on both iPhone and iPad? How is this possible, as the screen size should be varied according with UI de开发者_运维知识库signs. I am not sure about this. And also, I already created an iPod touch application using iPhone SDK 3.1.2, can I make this app a universal app?

Can anyone help me?

Thanks in advance

1. the idea of universal apps is to have just one app for both devices. The universal App can run two screensizes etc.

2. Upgrade your existing iPhone app to a universal application that runs on both iPhone and iPad. Customers buy Universal apps once and can install it on iPad, iPhone and iPod Touch devices. Learn more reading Apple’s documentation.

3. its documented ;-) usually apple provides allmost every bit of info, but it isn't always easy to get it ^

cheers Simon