Like many of you, I am completely addicted to Threes!, the lovely new heir to the Drop7 and TripleTown fortune. And by fortune, I mean all of my time.

David Wolinsky’s review hit the nail on the head, but as a strap-hanging commuter of New York City, I can’t help but feel like there’s something special about being able to play a game with one hand and hang on for dear life with the other.

It’s not a common use case in a sense–having one hand hooked to a steel railing–but in another, building for this type of experience puts Threes! in a separate category of elegant user design.

It dominates something called “The Thumb Zone.”

The phrase was invented by Steven Hoober, author of the O’Reilly book Designing Mobile Interfaces and as the name implies, the Thumb Zone is “the most comfortable area for touch with one-handed use.” Hoober found that 49% of users held their phone in one hand and used their phone with one thumb.

In an unflattering critique titled “Facebook Paper’s Gestural Hell,” Scott Hurff pointed me to the concept and he breaks down exactly why the Thumb Zone is so important. 

I’m a typical, bored user waiting in line for delicious burritos or coffee, I want to maximize my swipes / flicks and see as many stories as I can, as quickly as I can.

Replace “stories” with “moves” and you have an accurate assessment of how good mobile games should work. In fact, the ones I often spend the most time with on my phone (iPad is a different story) are the ones that cause me the least amount of pain. This may seem obvious, but it’s surprisingly uncommon. It still doesn’t explain Flappy Bird‘s ascent, but hey, let sleeping birds lie.