Apple has just announced from their iPhone event that all 1.3 million iOS apps currently available in the App Store will instantly work on the iPhone 6 and iPhone 6 plus due to smart scalability. There will also be tools available to developers so they can customize their apps for a better experience on the larger screens.
Essentially this means that unlike the scaling issue iPhone 5 owners experienced with apps that weren't yet made for the larger screen, all apps will automatically scale to the correct resolution. If developers choose to use reachability features that Apple offers, they can do so for the larger models.
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