The best touch gestures to use are the ones the user already knows. However, these are in quite limited supply. Because of this, the next best gestures are those that a user can be assimilated into the user's touch vocabulary after only a single use.
To illustrate this concept, let's consider some terms from modern philosophy.
A priori and a posteriori are two Latin phrases heavily used in philosophy—particularly epistemology, the study of knowledge. In essence, something can be known a priori if it can be rationalized without knowledge of a specific world. Normally, this includes various logical statements and mathematics. A posteriori knowledge, on the other hand, requires direct experience with the world to know.
When thinking about touch gestures, these two concepts come into play. There are the gestures that you would rationally assume would work before touching a device and gestures that must be learnt. (This is a slight departure from the strict definition of the two terms, so please excuse my philosophical sloppiness.)
There are a couple interactions you would rationally expect to be touch gestures, based only on understanding how objects in the real world work. (These are a posteriori realizations from the physical world that are ported as a kind of a priori to the virtual world).
That's a pretty short list. Things like double-taps and two-fingered dragging don't even make the list. Were I more generous—like Descartes, who postulated multiple a priori proofs of god from his armchair—they may even be included. However, they are still learnt. Seldom does double-tapping work in the physical world.
Yesterday I was at a Microsoft Store. A real one. I was curious about the hinges on the new Surface tablet, after watching a video that talked about the sound design of the hinges. I wanted to see if closing it was really like closing the door on a Mercedes. (Spoiler: It really is the tablet equivalent).
Anyway, I find Windows 8 fascinating and intoxicating.
It's cool because it's the first real overhaul on the windows style operating UI that Xerox came up with in the late Triassic (70's ... whatever), and the first true competitor to the iPhone-based app-centric UI model for mobile devices. Of course, earlier devices, like the Palm ones, had the same app-centric model.
So, I opened an app on the Surface.
"How do I close it?" I asked the extremely helpful salesman. I had briefly tried the iPad close gesture of all five fingers stretched out and then pulled closely together. This didn't work.
"It's kind of strange. You touch the top of the screen and drag downwards," he replied.
I considered this for a moment. It's strange in that it would be hard to discover. It certainly wouldn't be something that a user would consider before touching the device. Certainly not a priori. But on the other hand, it's not strange at all.
The gesture immediately makes sense.
As an occasional Windows user, I can map it to my Windows experience and add it to my Windows 8 mental model:
With this model in place, I will remember this gesture probably forever. Even if I never use a Surface again.
The iPad close application gesture is similarly easy to remember. I touch all sides of the application (I spread my hand wide), and pull my fingers in. It is as though I am gripping the app and sending it backwards into the device, back behind the main screen where all my other app icons are.
These are good gestures because they can be simply taught and easily remembered. The list of gestures that the user can figure out before using a tablet is very short. The list of gestures they quickly learn (like two-fingered scrolling) is also quite short.
Therefore, the knack of developing good touch interfaces rests in coming up with gestures that map closely to our previous experience—either in the real world or on personal computers. These are the gestures that will, in time, make up a larger touch vocabulary.
Have a favorite or love-to-hate gesture? Drop a line to @honzie.