Watch people play games on their phones and you'll notice something: almost nobody is holding the phone with two hands. They're on a tram, holding a coffee, carrying a bag, pushing a stroller. One hand holds the phone; one thumb does all the work.
When we started Knot & Bolt, we designed it the way PC games are designed: buttons in the corners, menus at the top, information spread across the whole screen. Our first public playtest was on Lisbon's 28 tram. Seven of nine testers gave up within three minutes, and four of them told us the same thing — "I can't reach it."
The thumb zone is smaller than you think
On a modern 6.1-inch phone held in one hand, the comfortable reach of a thumb covers roughly the bottom 40% of the screen, curving up toward the side of the holding hand. The top-left corner, where most apps put a back button, is effectively a two-handed gesture for right-handed players.
Our three rules
After that tram ride we rebuilt the entire interface, and the rules we landed on have survived three games since.
- Every action lives in the bottom half. Undo, restart, hints and the pause menu all sit within easy reach. The top of the screen is for reading, never for tapping.
- The play area is symmetrical. Left-handed players are around 10% of our audience. Mirroring the layout is a toggle, but a symmetrical board means most levels don't need it at all.
- No gesture needs a second finger. No pinch to zoom, no two-finger rotate. If a mechanic needs it, the mechanic changes.
If a player has to shift their grip, we've already lost them for a second. Do that ten times a level and they'll put the phone away.
What it cost us
The rules aren't free. Lumen Drift's mirror rooms would have been easier to read on a landscape screen, and there's a version of Pocket Orchard with a richer inventory menu that we cut because it needed the top bar. Constraints remove options.
But they also gave each game a shape. When the whole interface has to fit under a thumb, you stop adding things that don't matter. Our support inbox has never once asked for more buttons.
Next time you're on a bus, look around. That's who you're designing for.