On a standard twelve button numeric keypad, there are 46 pairs of orthogonally adjacent keys:
11 12 14
22 21 23 25
33 32 36
44 41 45 47
55 52 54 56 58
66 63 65 69
77 74 78 7*
88 85 87 89 80
99 96 98 9#
** *7 *0
00 08 0* 0#
## #0 #9
Orthogonally adjacent are easy to type. On a touch screen, they can be swiped.
A keyboard can be depicted with multiple symbols per key: the center symbol is what you get if you repeat the number, and the symbols on the sides are what you get when the second keystroke is one key over.
Flit keyboard
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