All sequences are belong to us

It's easier to reason about the possible combinations by laying them out in columns and rows. The visualization also allows us to make an intuitive connection to the classic QWERTY layout.

With one key under each finger, there are 10×9÷2=45 ways to pick two out of ten. But for comfort, it's beneficial to have one hand only press one key at a time; each hand pressing one out of five keys produces 25 combinations.

These combinations are partly useful. A 5 by 5 grid covers half of the QWERTY layout, and keeps its arrangement.

To type a letter, first simply pick a column, using the same finger one would use with a QWERTY keyboard.

For example, to type the letter V, in the fourth column, first push the 4th key/finger (the left index finger).

The rows are mapped to the other hand. For letter V, finger 6, the right thumb, should be used.

One can recall which two keys should be used together, by summoning up this image.

Similarly, we can type a letter on the right hand side.

Now selecting a column means use a right hand finger first; then, use the left hand to select the row.

For instance, press finger 7 (right index) followed by finger 2 (left ring) for the number 7.

The two halves are distinguished by the order of presses, to type a letter on the left, use the left hand first; and vice versa.

It doubles what could have been done with only one 5 by 5 grid.

We can go one step further, and once again double the number of possible combinations, to 100, without needing extra key strokes.

For instance, consider the combination involving pressing finger 3 first, followed by finger 6, the two can be released in two ways:

• FIFO order, or first down first up order

  That is releasing finger 3 first, the combination can be noted as 3,6,3,6.

  One may use it for typing letter c.

• First down last up order

  Namely, 3,6,6,3, release finger 6, then release finger 3.

  Something different can be configured to happen, like typing the letter C (in uppercase), or typing ç, or emulating Ctrl+C.

New forms of input devices are made possible. With only several keys(switches) required, portable inputs can be made small, without making keys tiny.

We can go in the other direction, for example, by making large buttons for arms, one can entirely avoid fatiguing fingers, Alternatively, the input device may not look like a keyboard at all, such as: arm-operated joysticks.

A portable version built with joysticks, similar to gaming controllers, is shown here.

One can make use of existing hardware: only software is needed to implement sequences on a touchscreen.

When using a full PC keyboard, here are some ideas for making additions that can be useful. For example, instead of pressing Ctrl+T for a new tab, hold N and tap T; instead of pressing Ctrl+Z to undo an action, hold U and tap D. This way, fingers don't need to make an effort to reach Ctrl, hopefully they can get more rest and less RSI.

But we can also do more with only ten keys, so large keyboards may not be as irreplaceable as they used to be.

Combining two near-simultaneous taps we have the QWERTY covered with ten keys. But these combinations are only a special case of generalized sequences.

We can have three overlapping strokes in a sequence. They don't need to be all held down at the same time, a sequence is not broken as long as there's at least one key being pressed. So the first or second pressed key can be released before doing the third press. This reduces finger strain, and we can also make use of the ordering of events.