Apple's iPhone Duo goes on sale this Friday, but one software feature will not be ready at launch. According to Apple, Reachability will arrive on the device "later this year."

Reachability is not new to the iPhone. Apple first introduced it in 2014 with the iPhone 6 and 6 Plus, as the company's phones grew larger than ever. The feature was designed to make it easier for users to touch elements near the top of the display without shifting their grip.

On current iPhones, people activate Reachability by swiping down at the bottom of the screen. The gesture slides the interface downward, moving content vertically closer to the user's thumb.

The iPhone Duo is expected to take a different approach. Apple reportedly plans a new version of the feature that brings content within reach horizontally rather than vertically. The aim is to let people with small hands more easily get to the left or right edges of the screen with a tap.

The addition responds to a concern some prospective buyers have raised about the device. Both the outer and inner displays on the iPhone Duo are wider than the screens on other iPhones. The interface also clearly favors right-handed use, a point 9to5Mac's Chance Miller confirmed in his review of the phone.

For anyone who finds it hard to reach interface elements along the left or right edge of the Duo, the forthcoming Reachability option should help. Until it arrives, buyers who want one-handed use will have to wait for the software update Apple has promised for later in the year.

The phone's full review is available from 9to5Mac, which also asked readers whether they are worried about one-handed use of the iPhone Duo and whether the planned Reachability feature eases those concerns.